Looking for recommendations for tactical games

I'm not sure if it covers the customization aspect as much as you'd like (its strategy layer is a bit more simple), but I found Valkyria Chronicles to have a very similar feel to XCOM: Enemy Unknown/Within, tactics-wise.

Another one is Darkest Dungeon, thought that one has unforgiving perma-death with only auto-saves so I don't know if it'd be considered too hard. ;D (And now that I checked you've already played it, so why am I even mentioning it?)

Hmmm, I can't really think of any other hard tactical games. The King's Bounty games are sorta close-ish and have a lot of freedom on how to approach things (or at least Armored Princess/Crossworlds has; the later games were... underwhelming)... but if I can cheese it on the highest difficulty where every enemy is marked as "Impossible", then it can't really be all that hard. XD

Oh, and don't worry about XCOM 2- it's 100% the buggy and unoptimized game's fault that it's not running on your computer.

Not sure if you are going to like these or not, but here's some of the games from my library I think may fulfill your request (just make sure to double-check them before trying):
Codex of Victory
Hard West
Regalia: Of Men and Monarchs
Shadowrun trilogy
Divinity: Original Sin 1&2

I definitely suggest Shadowrun games!

I see from your library that you know Battle for Wesnoth. Would you try it some more?
It's more of a turn-based strategy game, but in the campaigns it does have unit progression, with some choices, and usually you can recruit new kind of units while you progress.

The Banner Saga games and if your rig can handle it: Mutant Year Zero.

Chroma Squad is quite fun too, albeit not as deep.

The warhammer 40k turn-based tactical games like Sanctus Reach or Chaos Gate.

Xenonauts

I think Invisible, Inc. checks these boxes.

If you are okay with a fantasy setting and RPG elements, Fell Seal: Arbiter's Mark may be interesting.

If you don't mind playing another XCOM game, I would probably recommend the older X-COM games, like X-COM UFO Defense (in Europe known as Enemy Unknown) and the sequel Terror from the Deep. For their age they still look good and play well. UFO Defense can be tough, but Terror from the Deep is actually really hard. They are my favorites of the entire series.

i'm not a fan of tactical games, so i can't recommend any, but i hope you'll find games that you want :)

Xenonauts, Jagged Alliance 2, Fallout Tactics, Silent Storm, X-Com: UFO defense, X-Com: Apocalypse, Warhammer 40,000: Chaos Gate.

Shadow Watch

Not a big fan of RTS, but I had a blast with the original MOO games from back in the day. Your potato lappy should be able to play the first two games with no problems. Just don't try playing on the hardest difficulty unless you're not afraid to bust out the HEX editor. 😛
Just an FYI: MOO3 plays like a damn encyclopedia, so I'm not going to recommend it even if your laptop can run it.

+1 to anyone recommending Jagged Alliance 2.

Try it vanilla for maybe two wipes, then add the 1.13c mod (assuming that's the latest -- it was te most recent update of a community mod years ago).

The game doesn't teach you all the many many nuances very well, so you will wipe, so don't spend too much time getting invested in character creation at the start, but it's worth the experience trying the original vanilla version first.

(Sci-Fi mode on too is fine. I still consider that part of vanilla, and the commentary from your mercs is fun.)

Not really close of what you're asking but... have you heard of Fire Emblem?

Banner of Maid, Space Hulk: Tactics, Numantia, Space Hulk Ascension, Wargroove
God Wars The Completed Legend, Warmachine Tactics, Valkyria 4 Chronicles
Frontline: World War II, Field of Glory 2, Afghanistan '11, Frontline: The Great Patriotic War
汉武大帝传, Warring States, Fantasy General 2, Canon - Legend of the New Gods
Lock 'n Load Tactical Digital.

Still got a lot. If you want, I can list out more.

I usually stay away from microtransaction but I enjoyed played mobile game called arknights.
There's a sub class called summoner that looks super fun in fan showcase but I'm too smol brain to use it 😅

Maybe PhantomDoctrine would interest you. This one has silly investigation mechanics, but the tactical gameplay isn't too bad. I wouldn't say it's as fun as XCOM, but it's something, you know.

I liked Fae Tactics.
Crowntakers
Eador series(can only speak about genesis though) Eador: Genesis, the New Horizons mod is a very nice addition. Which is a standalone download, you don't even need genesis to play it.
Renowned Explorers: International Society
Maybe Ash of Gods: Redemption, haven't played it myself yet.
Wargroove

3rd aspect... is up to you and not sure about 4th aspect as I can run the games just fine, but here goes some recommendations:

• Warhammer 40,000: Mechanicus is a bit on the basic side, what you see is what you get, but was still fun.
• TROUBLESHOOTER: Abandoned Children has a LOT to keep you busy and more gets added regularly.
  Shadowrun Returns -> Shadowrun: Dragonfall -> Shadowrun: Hong Kong in that order.
  The newer games use a better version of the engine the older ones use and there are workshop campaigns/maps to try (can even play Returns in one of the newer engines.)

I feel you Commander ; )
My top recommendation would be Dreadlands - Indie Gem (90% polished, has an extensive Demo!) closely followed by Tainted Grail - Card Battler Roguelike with a lot of customization going on.

Oldies but goodies:
Hard West
Satellite Reign - though not turn-based
Massive Chalice - smtg. a little different
Warhammer 40k: Spacewolf

Wild Cards !?
Mutant Year Zero
Phoenix Point

As a last resort, there is a great overhaul mod for XCom: Enemy Within called Longwar. Unfortunately there´s no workshop support for it on Steam, so you´d have to google & download it. Longwar 1 or 1.0 should be it. It adds quite some depth and challenge to the game and lets you play with up to 8 soldiers ..

Jagged Alliance 2, Silent Storm, Fallout Tactics

Phantom Doctrine, I haven't finished it yet but I'd say it satisfies the xcom itch and has enough new stuff to keep things interesting, for example you can finish most missions in stealth. Beware though I don't know how good the performance will be on low hardware.

SteamWorld Heist, it's basically xcom in 2D. Great game with enough variety in characters and abilities and the distinct Steamworld universe humour.

The Secret of the Menhir Forest, this one is from the creators of the Legend of Grimrock 1&2 ,which are great games btw, and it's set in a medeival fantasy world, you have rangers and wizards etc. I've only played it for a couple of hours some years ago so I don't know if it covers your preferences.

Attack of the Earthlings, humans land on an alien planet to mine resources, aliens get mad and try to kill them all. You play as the aliens. There's a skill tree to upgrade your aliens and evolve them into different units (ranged, tank, stealth...) and give them new abilities. Overall pretty fun game.

Crookz The Big Heist, haven't played it but I have it installed. It looks like an xcom game and has positive reviews so maybe it's good.

The Banner Saga trillogy. The combat is probably what you're looking for here, tactical and punishing with lots of unique characters but keep in in mind these are for the most part visual novels with a lot of dialogue and text.

I would also recommend you look into deck-building games, like Slay the Spire, SteamWorld Quest Hand of Gilgamech, Monster Train, Dicey Dungeons, kinda. I've found they satisfy the tactical gameplay itch and each one is unique with their own gimmicks and world.

Probably not what you are looking for but when i read the tittle i remembered a game i had a lot of fun with:
https://store.steampowered.com/app/258050/Survivor_Squad/