Hold up, let's forget most of your post and focus on this:

"I think low speed characters should be very overpowered in some other aspect, and this game doesn’t accomplish that like Smash does."

You have no idea how wrong you are with this statement. The heavies in smash are horrible design. In the earlier smash titles, heavies were never viable because they had crippling weaknesses. Heavies are big targets, which is kind of unchangeable, but they were too slow and had poor recoveries. So what did they do in Ultimate? Fix or help protext their weaknesses and balance the characters? Nope! They just made their strengths even stronger and polarized them further. They're better in the metagame than they were because of this, but they're miserable to play against. For most of the game the heavy player doesn't get to play, but if they guess right a few times or their opponent makes a few mistakes, they just kill you. That's high variance gameplay that doesn't belong in a competitive game. You can't make DK and Bowser smaller, but there's no reason they can't have good recoveries. There's no reason they can't have projectiles to help compensate for their lack of fast run speed, or movement tricks to help them approach and land (imagine DK with Diddy's Monkey Flip). Don't make them stronger and heavier with more super armor! Ew.

You can't create a balanced real-time action game without speed becoming the most important attribute, because these games are fundamentally based on movement. Movement and positioning are the things that the player can control, and it's what separates good from bad players. In Smash, they needed to give slow characters alternative movement options or projectiles to compensate. In MSBL, there's no clear answer. Fast characters dominated MS Charged too, and you couldn't even customize so half the cast was unplayable. The high-tech Rosa meta at launch in this game was stupid, way worse than what we have now. If you want technique to be more valuable, maybe we should increase the gap (make perfect moves are extremely difficult with low technique) and if you want speed to be worse, they'd need to decrease the speed gap (low speed characters aren't much slower than high speed characters.) I don't want fatass max strength Warios who can score from midfield running around, that would be cancer.