I'd say the reason I chose them to main was because of their speed and smash attacks with Mythra being quick and getting good percentage pretty quickly and Pyra having strong smash attacks to finish them off while Little Mac has the speed and gets good percents all around while having super armor on smash attacks
Interesting that both characters you enjoy most are both mediocre at recovery with little mac being the worse obv ... So this tells me you're valuing dmg output, wiff punishing and get off me moves more so than off stage play. If you want similar characters there's tons, though because you use the literal top and bottom characters (but for the same reason) just about any of these will have some elements that are similar to those two's kits but likewise will not be those 2 so will be missing elements that you've come to rely on.
So for example..
Ok, Little Mac hits like a truck. Well you can find super armored moves in Ike, Ganondorf, and Bowser.
Or Byleth..
But with maybe the exception in Ganondorf all those characters also have off stage play as part of their kits and it's not something you can ignore so you may need to exit your comfort zone a bit.
To me your Ultimate (ha!) Choice is Kazuya. He can do everything Mac or Dorf can do but even better and he has a ton of other options they miss.
Aegis ... Maybe try Roy or Shulk? Bayonetta? IDK that's a tough one to top.