I'm glad to see that there's still people working on improvements to BrawlEx despite Sm4sh Wii U and Ultimate being out for quite a while now. Even though Brawl is an older entry, it still has untapped potential.
Yeah, Brawl is the most modifiable entry in the series. I've seen that Sm4sh has a few custom movesets and plenty of model imports. But I haven't seen any custom animations. Ultimate seems to be about as modifiable as Sm4sh, albeit with less mods as it hasn't been out as long. Melee seems to be very inaccessible with just some animations swaps and custom movesets. It has very little model imports. I've never seen any mods of 64, but I'm sure it'd probably be in the same place as Melee.