I think Dominus Felis went about giving advice in the worst possible way. Never put somebody down. He is trying to learn and get better and he is seeking out advice. The Smash community as a whole is supposed to be extremely welcoming. Honest, yes, but for someone seeking advice on how to improve, you never just bash them and tell them how bad they are. You were in the same place once.
Goldylox, as much as I disagree with how Dominus went about giving advice, I do agree with what was said about shines. Shines are Fox's best tool. I saw a lot of the time you would do stuff like d-air into a grab and you'd miss the grab because Marth would be behind you. Instead of going for the grab, drillshine. You've got to L-cancel the d-air and immediately shine to drillshine perfectly. The grab only accounts for what is directly in front of you and only covers one option. Shine accounts for what is directly in front, on top, and behind you. So even in that situation alone, that's two more options covered! When people started talking me through optimizing my options and how to do so, I improved leaps and bounds. Another thing I would would on for sure is short hops. Short hops with Fox are so important. Especially with n-airs. You love to use n-air, but rarely did I see a short hop incorporated. I'd definitely work on doing SHFFL-ing. Short Hop Fast Fall (insert aerial move here) L-cancel. It's extremely helpful and effective in speeding up Fox's movement game and Fox's movement is SO incredibly important. I think you're definitely on the right track, but there's always more to practice.
To practice:
SHFFL
Dash dancing
Wave dashing
Dash dancing with wave dashing incorporated
Wave shines
Wave landing on platforms
Invincible ledge dash
Running shine into wave dash into dash dance
You've got this, man. Keep the practice up and you'll get there! Remember, the GOAT himself is still practicing.