RPG as game genre is stupid!!

Nah. Diablo is more Hack & Slash less RPG.

tbh you should probably play one of these games before commenting on what you think the term RPG means. You are missing a whole face of RPGs, and arguably the one which has the better claim of actually being RPGs, seeing as most of them are based on the most popular tabletop RPGs (Dungeons & Dragons). (Not saying you should like them, I don't like them myself).

Similar like night and day.

Diablo is an ARPG [Action RPG], and a direct derivative of the Roguelike format, meaning it also inherently classifies as a Rogue-type/Roguelite.

Baldur's Gate, Pillars, and Planescape: Torment classify as CRPGs. Yeah, it stands for "Computer Role-Playing Game, which is nonsensical, but blame the fact that it was the first western labeling for RPGs [ie, during a point where it was more important to distinguish digital RPGs from non-digital RPGs than it was important to distinguish from then-non-existent subgenres], and thereby signifies "traditional" western RPG format. In one sense, one could also consider "Ultima-like" as a synonymous labelling.

There are also several other xRPG labels, such as JRPG and TRPG. Though TRPG [Tactical RPG] is rather confusing to some, given that it's SRPG [Simulation RPG] in Japanese labeling (but ONLY in Japanese labeling or within conversations among those familiar with such; In English, SRPG stands for Strategy RPG, instead).

Given the large variety of labels already established, assigning simple descriptions to specific games is actually rather straightforward, so long as both you and your conversation partner are familiar enough with subgenre labels to be able to use them to quickly distinguish mechanics.

Meanwhile, Witcher and Skyrim (as well as games such as Gothic and Zelda) both classify as Action-Adventure RPGs (more specifically, 3D Open-World Action-Adventure RPGs).

You seem to be coming up with new labeling in your OP, but the labels for distinguishing RPG subgenres are already out there. (Meanwhile, if a game is similar to an RPG, but primarily a different genre, one would say something along the lines of "It's an FPS with an RPG-like leveling system". Though now that leveling systems have become fairly standard across most genres, one more typically just applies the "Upgrades" label to such a game- or, in casual speech, "It has an [ability] upgrade system".)

If you've got a favorite subgenre (or subgenres) of RPG, then it's as simple as familiarizing yourself with that specific label, and referencing off of that. Familiarity with the other labels will pretty much only be important during the types of conversations noted in your OP but, at that point, you can just look up the label for that game [and, if your friend is unfamiliar with the label, then you'd be needing to go into additional details regardless, meaning you don't really lose anything from where you would have been to begin with].

However, as far as the broader "RPG" label, that's going to always be more vague, given that it's a top-level label. In fact, compared to RPG, other top-level labels can be even vaguer:

Platformer, for example, easily indicates that platforming (which, like upgrade mechanics, is commonly utilized even among games focusing primarily on a different genre label) is heavily emphasized. It does not, however, give you much insight into anything else about the game.

As a further (and likely the most egregious) example, the Simulation label covers such an absurdly broad range of subgenres that play out completely different from one another, that it basically always has to be given additional clarification of intent.

Oh! Those are pretty important, too. I added the ", etc." at the end because I felt like I was missing something. These play a huge role in making an RPG game, well, a game. Games that have a story that you follow (And a choice or two.) without gameplay aren't games to me. They fit more into my definition of movies or interactive VNs.

So if there was no leveling or skill tree, you would still call it RPG?

No. If shooter (like Prey 2017 I play now) has ability tree I will call it FPS with RPG elements. But never RPG.

On the other way around - you an have character where you can't customize much, apart from initial name. And it will be RPG for me, as long as you have non-linear story with choices, where you can engage. And it's not visual novel ofc, as VN is more like clickable book.

I meant stuff like the Fallout 1, 2 & NV character creator.

To be honest, I have no idea it may have been an Atari game or a MS-DOS one...I was too young to remember.

For me it is strategy and RPG as you as a commander must take care of everyone life and it makes me to try and roleplay the best commander that will save everyone he can etc. Things you pick, ways you build, upgrades you choose will make your soldiers survive or die in the field and also I like to name one soldier after myself and try to keep him alive as I think good commander would be also in the field with his people.

Disco Elysium would!
but telltale feel more of the interactive movie kind, very little inputs from the player i have trouble calling it a game

That's hard to say. I normally wouldn't have thought of them as RPGs, but I think in Life Is Strange you can make a lot of choices that affect your character and the story. I probably would think of it as an RPG.

I don't know about every single game of Telltale's, but it's not a far stretch, at least for its elements. Though TT kinda sucks at depth of choices.
About Final Frontier I've seen people complaining about Gabriel (I think) and how much of an insufferable dick he is, while in my game he was eager to help, quiet but present and never a hinderance. This means that at least regarding that character, it really did matter how I approached him, having the choices and consequences - like how in other games being polite or rude is a roleplaying choice, generally you can do that in TT games as well. Just... sucky consequences, guaranteed progression and the jazz takes away from the impact of it.

The aspect they lack which is other important part of RPG is character development(as in stats)... So for me they don't really qualify... Also free player agency isn't really there.

That's why RPG is called Role Playing Game, you have a role and somehow you shape the game story as you see fit according to your character. Do you want to be a bad or good person? Do you want to get something by killing, stealing or use diplomacy? Story is important but shaping it id better. A game with only story is a pure Visual Novel.

Game mechanics are much more important on a RPG game, it makes you think and analyze the situation that you're into. Turn based isometrics you have to think on your movements and the best way to win a combat without losing too much health (saving them for the next fight). First person you need to master the best way to fight with melee or if it's a shooter, use the weapons appropriate to the current fight.

Another example of First Person RPG is the old Ultima Underworld. I loved that game.
Eternal Ring also is an incredible game.

I didn't played Disco Elysium yet, looks like a great game.

Those are good examples indeed! Mine were about the bad :D

I think it would be more accurate that I hate when a game adds RPG elements for the sake of ticking another genre-box, while doing nothing that really changes anything, like a bunch of 1-3% bonuses. Working out your weaknesses, or building around your strength and preferences in Bioshock with weapon upgrades, AND especially plasmid upgrades changed so much how a combat encounter could play out!

Borderlands 2 - FPS / Looter shooter

DARK SOULS™ II - Souls game. This one has it's own genre by now. High difficulty bosses and tight combat

Darkest Dungeon® - Dungeon crawl

Divinity: Original Sin 2 - Definitive Edition - RPG

FINAL FANTASY X/X-2 HD Remaster - jRPG. A lot unnecessary grind

NieR:Automata™ - Action / story

Stardew Valley - Farming sim

Sword Legacy: Omen - don't know this one

The Elder Scrolls V: Skyrim - RPG / sandbox

The Witcher® 3: Wild Hunt - RPG

I'd think only about Witcher, bc I'm not fan of isometric RPG (like Original Sin), and Skyrim is really "watery"

TBH "indie pixel art great soundtrack" would probably grant a game a place in my wishlist if the trailer also looks nice