xbox-game-pass question

I think you can't resuscribe with a plan ongoing

You will have to end your ultimate sub and then resubscribe to the 4.99 package

After 1 month price is 15€ not 5€ (in Europe)
The best deal right now is remove delete all your payment methods from your accounts,
Delete the auto rennoval and subscription,
create a new account and get 3 months of ultimate for 1 euro
Activate family share (for play games of xgpu in new account with your old account)
If/when they ban your phone number and you is not able to make a new account just buy a new phone card that in Europe is 5€ and is more cheaper than 14€.

With subscriptions (eat,Ms,google etc) Is better to use "virtual credits cards" that you can create or delete instants with services like this..instead of your main payments methods
Is better to apply daily pay limits to these cards also f they is virtual and you will delete it just for be sure that they will take only 1€ and not 1 cents + (for example by fraud you with "365 office" subscription or other things like these

I assume you are talking about game pass for PC, yes you can do this

Depending on your plans* you should:

1. (not your case, skip) Buy the 1-month Game Pass (Beta) for 1€
2. Buy as many 1-month codes as you want (up to 15 in your case I think)
3. Disable automatic renewal
4. Add code for 1 month and say yes to "automatic renewal", it will give you an extra month
5. Disable automatic renewal
6. Repeat steps 4-5 until you redeem all codes
7. Buy the 3 months of ultimate pass for 1 euro

With 16 one-month codes I got 36 months of ultimate pass for an extra 2 euros: 1 month game pass (1€) + 32 months (2 x 16) + 3 months (1€).

Right now I'm good until November, 14th 2022...

I hope "Halo: The Master Chief Collection" will be included in the Ultimate Pass as expected!!

EDIT: I didn't notice that you were only interested in PC, what I explained above is the best deal if you want 3 years for both devices...
EDIT2: (*)If you don't know if you'll buy a console in the future, maybe the best approach is to fill your current Game Pass up to 33 months and then convert it to Ultimate. You'll get 36 months of Ultimate at the price of the basic Game pass. I guess you can go back to the basic Game Pass in three years (if we're still around).