I think forcibly retiring characters is a lame practice, and one of the biggest strengths of this game is that old characters can play together with newer characters much more sensibly than in most other systems where a few levels difference place people so far apart on the power scale that it really isn't possible for those characters to have fun together. I've seen 1000 XP characters that were entirely ineffective in combat and 200 XP characters that could dish out 50+ damage in a single round and had 8 soak. Overpowered simply isn't a pure function of how much XP a character has in this game.

The only thing you can really point to in the XP system that makes older characters not perfectly compatible with newer ones is that the progressive XP cost isn't anywhere steep enough to really let people close the gap naturally in the long run. Only increasing the cost of new talent trees by 10XP every time just isn't enough to cause a substantial slowdown in development.

If a character is way too powerful I suggest checking their gear above all else. I think the problem a lot of GMs run into in this game is that they have been trained by D&D that gear is an integral part of your character and the DM never takes any gear away from the players as a result. This game is very different. It specifically contains rules that are meant to damage and destroy gear over time, and you shouldn't feel bad about invoking them.

Money is way more overpowered than XP in this game by a huge amount. The second you stop treating people's gear as sacred and make enemies act with full awareness of what pieces of gear are dangerous and need to be neutralized you see the power gap shrink considerably. If someone lugs around a huge autofire cannon that clears rooms make the enemies target the gun. That's what a player would do if they are up against a boss with a super dangerous weapon that is much easier to destroy than the wielder.

Another huge thing to keep in mind is to use Strain and Stun damage. It's very difficult to build a character who is highly resistant to both stun and regular damage, and you should be taking advantage of that when you have someone pulling too far ahead. Make them take threat in strain more often, make enemies switch to stun to bring them down. It's a very easy to use but powerful lever as the DM to know what the weak pool is on every character, and then deciding whether to target their weak or the strong pool based on whether they are ahead of the other characters or falling behind in power. It's nearly impossible to make a character with two strong pools without being set back hundreds of XP just because there are never all that many pool increases in any given talent tree.

Another way to go is to give older characters responsibilities outside of what they are built for. Make the world recognize that they are the most powerful person in the team, and accordingly expect the most of them. Especially in the rebellion you can give them some command responsibilities and just start making them roll leadership all the time. Suddenly you're diverting their progression into Commander stuff and people can close the gap on combat. That's how a character should naturally progress anyways, picking up skills according to what they need, and it's not at all out of line for the Rebellion to order someone to learn something. It is a military after all.