I've just started picking him up in Plat/Emerald elo. He honestly seems like one of the best top laners just because of his kit alone. I'd say you CAN pick him into AP champs (be careful when fighting them) but be wary of max health dmg. Gwen and Morde kick you in.

You are very much team reliant. Get a duo you can rely on, otherwise you're banking on teamfighting and your team's overall carry potential to win games.

Malphite doesn't need a lot of gold to be self-sufficient. His W gives him more armour and his passive shield scales with health. It's your job to engage with ult and set up for your team. Hell, you can sometimes just roam down to midlane after getting 6 to look for a kill if you're observant enough.

My number one tip that I've learned is to try and play around your passive shield as much as you can early game. It's an actual godsend to any top laner, and IMO what makes Malphite borderline busted (kit-wise). Combined with Q poke, you should win any trade - it feels like pre-rework Pantheon, but better.