Ich habe dieses Lied auf Instagram gehört, und es hat so gut geklungen.

I'm not an expert, but as far as I can tell, putting a comma before "und" when connecting two main clauses is something you can do. That being said, we usually don't do that, and it looks a little odd.
In most cases, we don't put commas before "und" (often it's even wrong to do so), though there are exceptions where it's even required.

Es ist immer noch mein deutsches Lieblingslied, das auf Deutsch ist.

"noch" makes it sound like you expect it to change soon. Like "Das Auto ist schon sehr alt, aber noch läuft es." (the car is going it break down relatively soon tho)
If you'd like to say that it's still your favorite song, without that implication, you can use "immer noch" or "noch immer".

Ich möchte mehr Lieder wie essolcher Lieder finden.