You seem like a very reserved Sonic player compared to what you normally see. Or was that intentional in that match?
Was there anything in particular you were looking out for when playing that Fox? Like particular moves you could punish? Or were you reluctant to use any moves that you think Fox could punish? You seemed to be in pretty good control.
No, you're right: that's how I play in general.
The key here is to not let him Nair > USmash. I don't know if it can be avoided; but, I'm not aware if such an option exists so I rather just avoid the hit altogether.
Of course it's never a bad idea to just run up to them and shield.
... After which, you proceed to get grabbed... which can sometimes lead into Uair kills... Perhaps something to avoid, now that I think about it...
... Okay, I got it! It's best to approach with shield if he's facing away from you. Not unusual if they use Fox Illusion to run to the other side and they then try to stuff your approach with an utilt. If they're facing you; then, it's just up to you to bait out options, whatever they are. Spotdodges, grabs, jabs, USmashs, SHs, you name 'em.
Shield really will be your best bet in this matchup. It isn't like his aerials will be unpunishable. Best thing he can do is retreating bair... Which you may or may not be able to DA OoS if you're super fast about it. I dunno.
Be patient, too. I know it'll be tempting to go in if he's on the other side of the stage guns blazing (pun totally intended); but, smarter players will obviously not keep shooting past that point. They'll stop and see what you do. Don't be afraid to stop short yourself and see what his reactions are. Or if you prefer a little bit more action than simply standing in place, you can use auto-cancel aerials and even Nair if you're hoping to trade/catch him for a set-up from a weak-hit Nair. Just respect that Fox is kinda fast too and it isn't unusual to see one run in for a DA or USmash or some other crazy option if you get a little too predictable by sticking to one regime.
Let's see, you think there's anything else I can tell you that could help...? Hmm~...
I'm not sure. I'm speaking theoritically here; but, perhaps stages that have platforms are good? Think about it: he won't be able to just land on you with his own Dair/Nair/Etc. if he's going to be interrupted. And if you're sharking under the platforms, he'll have to reconsider. Of course, the same applies to you, so at that point, it might just be a matter of preference.
... Can't go wrong with Lylat, anyway. The first rule of Fur-******ry still applies somewhat; so, being there to disrupt their recovery path is lovely.