If this is your roster - https://swgoh.gg/p/843687536/ - then I see your problem.
Matchmaking is by GP as it was in GA but in GAC the total GP of your top {x} toons (and possibly top {x} ships) is used as your matchmaking GP. For this purpose {x} is the number of toons (and possibly ships) that you deploy on defense, times 2.
GP is a measure of resource investment and the higher relic levels add a considerable amount of GP.
You have quite a lot of characters at high relic levels which inflates your matchmaking GP and makes it more likely that you will encounter rosters which include a Galactic Legend.