Ok, of those games you mentioned, rating the games on netcode, gameplay quality (very subjective), single-player content quality (slightly subjective), and ease of learning (I think I’m good at parsing this):
Smash Bros is garbage online 1/5 netcode. Only good if you play offline vs CPU.
Killer Instinct is the best overall package. 5/5 netcode 4/5 gameplay 5/5 content 3/5 learnability. It is a pretty complex game, with a lot of unique features, but it’s so worth it.
MKXL is a version old, and MK11 is getting an expansion very soon, so I’d move onto that. 5/5 netcode 3/5 gameplay 4/5 content 3/5 learnability. Quantity of content is 5/5, but scummy practices make me knock it down a pag.
Soul Calibur VI is my best game 2/5 netcode 5/5 gameplay 3/5 content 5/5 learnability. Hard to recommend due to netcode. Soul Calibur , at least on PC , does have the most folks are masters right now though.
Tekken 7 2/5 netcode 4/5 gameplay 3/5 content 1/5 learnability. To play this game properly hurts your hands.
Blazblue, you’re a version or two behind on that. The anime game of choice right now is Granblue Fantasy. Guilty Gear Strive might be big in 2021, and Arcsys is finally making a game that might have good netcode
Skullgirls just had a small update, it’s getting a revival due to EVO online:
5/5 netcode 4/5 gameplay 2/5 content 2/5 learnability
Injustice 2 is very old and dead. Only playable for single player.
If I was to recommend something given your guidelines:
If you like NRS games, MKXI has a lot of bad players and great online, you could improve in that.
Granblue Fantasy might be a great choice, it’s simpler, if you can tolerate the shitty netcode.
Killer Instinct is a great package with limited XB1/PC crossplay (ranked is separate, player match isn’t)
(KI on Gamepass better choice than Steam for this reason)
Guilty Gear Strive should likely be your game of choice for next year.
I won’t sugarcoat it, fighting games are the toughest genre around next to RTS’s. 1 vs 1 so you can’t be carried, and 50%+ failure rates all the time until you reach a good level. It is frustrating, but if you enjoy it’s the most rewarding thing, to the point it’s hard for me to enjoy anything else.