So, you don't have to attack the opponent. Simple enough, but a key component of getting down when you're not being juggled. In that situation, wavelanding off the plats to ledge would be best as Bowser. When you wavelanded on the top plat, you waited a few seconds, then fell through to hit him with a nair. It's super obvious, avoidable, and punishable. In that situation, you could have just drifted away from him, without an aerial. Great offense is a good defense, but great defense is an even better defense.
In that set, you did well realizing that Zard's glair has the worst priority in the game. He was reliant on it, but you can jab him and kill him. But quit double jumping when you're on the top plat. You look like you're getting as much height as possible. Zard can just fly up and bair, and now you're without a jump. Also, DIing Zard's Uthrow: None. It throws at a weird angle, that basically causes you to go straight towards the corner, meaning no DI is usually best. In fact, since Bowser is so floaty, it may be better to DI a little down. Just hold forward. If you fortress on stage, you go to ledge. No ifs ands or buts. It's why you took a hard dair. Fortress has one hit on it, which is why you can actually hit them, but if you don't go to ledge, they can still punish you. I'm seeing a lot of aerials -> grounded move, even when there's no hit confirm. That worked from dair -> utilt a few times because he's bad or something, but on shield, no aerial is safe. If your aerial hits a shield, you instantly go defensive. Make sure to cross up behind the opponent to hopefully give you a few extra frames to defend. You did that a few times, which helped coming down. If you really dislike coming down so much - which is understandable against charizard - practice shield drop nairs. Overall your bowser needs work: some of your throws weren't the best option, terrible edgeguarding, punishes were subpar, other stuff I'm sure you recognize (nice ending).
To help with coming down from a juggle, which is a big issue against zard, but I didn't see much during that set:
Read up on SDI. I'm not going to be giving a big tutorial on SDI, but it's best use is when getting juggled. Whether it's Falco's pillars or MKs uairs, where you SDI in front, falcon's uairs, where you SDI behind, SDI is the single best way to escape. Our nair does help, but good jugglers won't let you out of hitstun. So, you SDI to allow yourself some more time to escape hitstun and get the nair out. Against zard, it's like Fox's uair, where you S/DI opposite the way he's moving.
I think I left a few trains of thought unfinished. reply with any questions.