This is my vision how Bowser could be more Balanced :

His Rolling Fortress is infinite, He can move around the stage as long as he want in this large, fast attack. To nerf it I would simply add a limit of turns he can do, 2 turns max means the player will have to get out of the shell before going outside of the stage cuz he'll simply be unable to turn back anymore. And also when Bowser comes out of his shell he should be stunned for like 1 or 2 seconds, so using Rolling Fortress isn't a free attack without downsides anymore (You could even add a little sprite of him being stunned ;) )

Same goes for his Fire Shot special, a limit of how many fire balls he shoot, like up to 3 and put a cooldown to prevent from people to spam it, cooldown could be between 5 and 10 seconds? Also related to this attack, his fire balls should be parry friendly, by that I mean it should be deflected back to Bowser when a player parry them, it will add a risk of using them cuz Bowser will take damage from his fire balls coming back at him for too much against someone who is trained to parry. (Also it could add a gameplay mechanic of Tennis if Bowser manage to parry back the fire ball coming at him)

I would change how the dash attack works, the problem is not about how much damage it does but rather "how does it damage the player" the fact it absorb and attach the player to Bowser like a black hole make it impossible to escape nor dodge.
I would make it a single hit attack instead of several little ones, pushing the players away from Bowser while dealing a single hit, just like Meta Knight Tornado move, single hit that push away the player. (Also maybe nerfing a little bit the range of this attack but I'm being nitpicking)

The Foward Tilt comes out way too fast imo, Bowser can spam it and as I said it's coming out almost instantly, slowing it down is good nerf I assume.

Just like the Fire Shot Special, def putting a cooldown for his Up Strong, Bowser can spam this large attack long as he want and he will always land it due to his large hitbox, simply add a cooldown of 5 and 10 like earlier, it will force players to use other attacks to hit someone coming back from upwards like Up Air.

I would add a cooldown to his Spike Ball to prevent spam just like the previous cooldown suggestions something around 5 and 10

Turning down his running speed slightly.

Giga Bowser's fire breath has been unavoidable every time I've ever seen it get used because it always stacks over at least 2 other moves that force you to run into the fire & the moment the fire stops, he breathes more fire.

I guess my main suggestion to fix that would be making it so he only breathes about 40% of the amount of fire he currently does so you don't get forced to run into a wall of fire that carries you into the top blast zone. Also, maybe make it so he can't just immediately breathe fire again.

The inhale tends to be unavoidable for similar reasons (the main reason being that if you're close enough to hit Bowser, you're too close to escape) but also does so much damage that it almost always kills.

could you Nerf giga fury bowser cus he keeps killing me because he does 100 damage for 1 attack and the BIG hit boxes make it impossible to kill him without losing a stock

Here's my perspective: his default moveset has a lot of range, knockback, and damage. Perhaps tone down the damage end a little bit, as the fire debuff (which is on a lot of his attacks) makes up for it anyways. Also, personally, not too much a fan of how massive, powerful, and spammable his specials are (except for up special, that one is alright). Maybe neutral special could have more endlag, side special could be made more similarly to Kragg's but without the jump canceling, and down special could be made a bit smaller (you literally can't avoid it if you're airborne). The rest of his default moveset is relatively alright, although it feels a little overkill that his attacks have so much hitstun.
Of course I also have a lot to say about the Giga Bowser form. As for the form itself, I don't think Giga Fury form should even be a thing, and his damage reduction is way too much (should be more than 1% each hit). As for his moveset, I really, really don't like how Giga form's down special can kill you at 0% with you having no way to avoid it. G-Down special should deal maybe 4 damage per bite rather than 10 and should have it's pull strength be heavily nerfed. G-Up special should prevent Bowser from attacking until it ends (maybe make it shorter) and should deal way less knockback. The rest of the form's attacks should just deal 75% less damage than they do (G-side special dealing 5 damage instead of 20) considering how fast and massive they are.