Welcome to the forums!
Congrats on your new Switch and AC!
For the iron nuggets, I'm not sure if you knew (I didn't when I first started), but when you hit the rocks with a stone axe, if you continue to hit it in quick succession, you can usually get about seven pieces of material from it. I would also recommend not breaking the rocks completely, as they take days to respawn. If you don't break them, you'll be able to mine them everyday.
Mystery island tours also generally have rocks on them too.
When I first started, I found the game quite slow to progress because I didn't know these types of things! That being said, if you have Nintendo Switch Online, I'd be happy to bring you iron nuggets!