Ian choosing to stay in prison with Mickey in s10 (even though it didn't end up happening) is such a huge act of love that people ignore.
Ian had every reason to want to get out (while Mickey had far less, so it wouldn't have been as much of a sacrifice for him if it were the other way round). He had his family waiting for him and Lip's new baby, while Mickey didn't have any family on the outside like that (except for Sandy, but it's hard to know how close him and Sandy were throughout their lives since she's such a new character). Ian had ambitions and goals and dreams for the future while Mickey (although seeming less hopeless about life than he did in the early seasons) only wants to survive, be with Ian, and have enough money to live off. Ian wants to restart his life after prison with a legal job and a steady income while Mickey is still happy with stealing and scamming and dealing when they get out (their arguments in s11 about money stem from Mickey being happy with working just not legally, while Ian won't let him do that anymore).
Ian also knows Mickey very well. As much as Mickey says that they'll be fucking other people if Ian's on the outside and Mickey's inside and says they won't wait for each other, Ian knows that doesn't mean they're breaking up. It hurts his feelings, definitely, but Ian is aware that Mickey's just insecure and unhappy and worried about Ian leaving. He knows that if he gives Mickey enough time, Mickey will eventually have a proper talk with him about how things will work, because that's what always happens. Mickey shuts down, is reluctant to communicate, pushes Ian away, and then eventually relents when he realises Ian isn't going anywhere.
All of that makes what Ian does in s10 that much bigger. When Ian chooses to stay in prison for Mickey, it isn't to save their relationship, because Ian knows they'll stay together. By now, he's very much aware that Mickey isn't going anywhere. (Besides, I also think that whether Ian was outside or inside prison, by that point he wouldn't let Mickey go anywhere).
So when Ian chooses to stay, it is literally for the sole purpose of making Mickey happy. He doesn't like that he's made Mickey feel insecure about what will happen on the outside and he doesn't like that Mickey's upset about him leaving. So instead of waiting him out like he might have done normally, so that they could talk, Ian prioritises Mickey's happiness over their relationship. Remember Ian wanted a 'temporary seperation', which him being on the outside while Mickey was in would have granted (even though it's not an ideal situation and would have been a lot longer based on what they knew of Mickey's sentence at the time). That 'temporary seperation' probably would have been really good for their relationship.
He does it purely to make Mickey happy. No one's life is at risk. They won't be breaking up. He just sees that Mickey isn't happy with Ian getting out and he tries to fix it. He even says it:
'If I choose it, you would be happy. I just want to know, yes or no, would you be fucking happy?'
And sure, he's pissed about that making Mickey happy. Ian's not happy with Mickey wanting him to do that, Ian doesn't want to do that, but he does want to make Mickey happy. He's willing to make that huge, huge sacrifice to stay in prison because it would make Mickey happy. That's it. It's not for anything else.
That's such a huge sacrifice and it's a massive gesture of love and I'm so glad Mickey wisened up a bit and decided that he was being pretty selfish and it didn't end up happening.
But Ian was going to do it. He was going to do it to make Mickey happy even if he was unhappy, even if he would lose a lot of time with his new nephew, his family, even if it might make it harder to get a job in the future, even if he had to postpone starting his life again. He was going to do it to be with Mickey and to make him happy and that's so sweet.
Side note: if anyone's seen The Good Place, it really reminds me of one of the last episodes with Chidi and Eleanor. When Chidi needs to leave the Good Place but will stay at his own unhappiness to make Eleanor happy and Eleanor realises that she's being selfish and has to let Chidi go.
It's like, they're just so in love. I hate when people say Ian doesn't love Mickey that much or whatever because did you open your eyes at any point during the show?? People really see Ian giving up so much JUST to make Mickey less sad and go, oh no, Mickey loves Ian so much more blah blah blah. Like shut up. They love each other the same.
Also, Mickey getting Ian to leave in the end shows how much he loves Ian. It's the same thing. Ian leaving would make Mickey unhappy but he lets him go in the end because he knows it's the best thing for Ian and it would make Ian happy. He puts aside his own insecurities and past hurt surrounding 'waiting' when he's in prison and Ian's outside to make Ian happy. That's also a big gesture of love, and it shows growth, because Mickey wasn't able to make that same gesture at the border.
(Maybe the border isn't the best comparison, because that break up would have seemed very permanent to them at the time, while the prison thing is temporary.)
Anyway, I'm just trying to say how much this episode shows in terms of their love for one another and the sacrifices they're willing to make for each other. I feel like people gloss over it a lot, because it came to a fast resolution for Gallavich (in that they made up in two episodes instead of over the course of a season like they usually do, and actually made up properly with mutual 'I love you's and an agreement to stay together which they've never done before), but it's SUCH a big deal.
It's a huge moment for them and they love each other so so much and they want each other to be happy more than they want themselves to be happy. They're both somewhat selfish characters (not a bad thing) and they're selfless for each other and that's massive. I just think it should be spoken about more.