I tried to analyze it as well as I could, but I'm no expert in the match-up and definitely not as good a Marth as you, but who knows, I might have seen stuff you didn't think of.
1st Game
0:32 Make sure to angle your shield up and towards, Ganon can't really shieldstab from underneath when you're on the same level.
0:36 I think Kage teched towards you on purpose there, knowing that you'd be caught offguard and try something like a fair again. Uair or maybe Nair might have been better choices.
0:50 ~ 0:55 I feel like the whole thing happened because you decided to land right above him when he was idle. Like against Falcon, you gotta respect their anti-air game, it often gives them a free ticket through your spacing.
1st game : I think you got punished every time you tried to approach with a Nair, be it because he shieldgrabbed or because he just outranged you with stuff like Fair. You could try to mix it up, like , going in then back out to bait one of Ganon's slow hitboxes and only then going back in with the Nair. I can imagine that being hard against players like Kage though.
2nd game :
5:42 That roll was probably a technical mistake, too bad it happened there as it could have been the best moment to take a substancial lead.
6:00 I'm not sure what you were going for with the sideB, so I can't say much about it.
6:23 You should do that nair to poke his shield with a short hop instead of a full one. That way you can try to push him off the platform while staying safe, and if you actually manage to and he enters that falling animation, you can immediatly follow up and keep it going.
6:42 I think Kage jumping right away after getting hit with the sideB was really likely. I would have gone for a second sideB right off the bat, but I'm not actually sure whether he could have made it out or not. (SideB > SideB > Weak upB > Dair or something)
6:55 Going at a Ganon right from a platform when he's just waiting for you is a pretty bad idea imo.
Round 2 : You did much better and it seemed you had much more confidence going into that round. The first two stocks especially were impressing, and this time your nair approaches all got in because you overshoot them a bit more, and he couldn't jump back > Fair anymore ( when he tried he got hit).
Hope that helps somehow.