Can I play The Division 2 with friends after Game pass subscription expires?

You would still need a Gold or Game Pass Ultimate subscription to play The Division 2 online. Gold is still a thing. It's just no longer required for free-to-play games. https://news.xbox.com/en-us/2021/04/21/online-multiplayer-free-to-play-games-unlocked/

Game can be removed for game pass after a while or may stay in game pass forever. But if it is removed you can no longer play it. Also, if your subscription of game expires, you can not play it. But you can play it again, if you subscribe to game pass
again, and if it's still part of game pass.

Anything you download from Game Pass, you can play while it's on Game Pass and while you have a subscription. Once it's removed from Game Pass or your subscription expires, you cannot play the game unless you purchase them game. If you purchase a game
while it's on Game Pass, you do get a discount on the game and you can continue playing it after your subscription ends or it leaves Game Pass.

And to add to Smwutches, you can manage your Game Pass subscription through your Account Settings and turn off the auto renewal and simply repurchase Game Pass for PC after your current subscription expires.

If I were to buy 4 separate 3 month subscriptions to game pass ultimate at the same time, could I apply each one after the previous 3 month subscription expires? I didn't know if the codes would expire.

As far as I am aware (I don't know for 100% certain), you can apply all 4 at the same time and they will stack up, thus giving you the 1 year, as you can have up to 3 years worth of Game Pass active. So, for example, I had 2 months of Game Pass left on my account when I recently bought my Series X with All Access which comes with 2 years of Game Pass included....so now my GP expires 2 months after the All Access version expires ie end of July 2023 instead of end of May 2023

From the Terms for the console version:

"Maximum 36 months of redeemed Xbox Game Pass Ultimate per account at a time"

Also in the FAQs:

https://support.xbox.com/en-GB/help...age-subscriptions/xbox-game-pass-ultimate-faq

Personally, I was redeeming Rewards points for GPU and regularly redeemed even when I had a month or 2 months left and never encountered an issue.