stuck in endgame

You're up a full exchange so you're definitely much better. You don't have to worry about his passed a pawn right now and likely not for a long time because his a pawn has to stay where it is to protect the pinned b6 knight. Your pieces are better placed than his. You rook and bishop both attack the pinned b6 knight, and his dark bishop isn't doing anything particularily useful. Also your comment about his queen going to the long diagonal with ease is really worrying about ghosts, because his queen is not going to go on the long diagonal anyway because you're not especially weak there. If he has any chance it's to play on the weakened dark squares around your king.

That was the more general stuff you were asking for. As far as specifics, black has an obvious threat of Qd3 with the forking of your two pieces. But you can stop that threat and win even more material.  Qh7+ followed by Qxf5 guards against that threat. You'll be up the exchange and a pawn, which should be winning.

I should also add that you should not just play Qxf5 in response to any move he makes. For example if he made a bad king move like Kg8 you can play Bxg6, or if he plays Qe7 or Qc7 then it's an easy win after you trade queens and play Rb5. So even if Qxf5 is your main idea, if he makes a bad move you can change your plans and go for even better lines.

Why not? 38.Qh7+ followed by 39.Qxf5 just ends the game.

Yes, that's exactly why I suggested it! Of course it wins. But I can think of moves for black after Qh7+ where Qxf5 is not the easiest way to win. Qxf5 is the natural follow-up to most black moves, but like I said there are some moves that black could make where it's even easier to win by playing something else. Such as Kb8 where white plays Bxb6 and after Qe7 trading off the queens leaves black without any hopes of counterplay and the pawn falls anyway. So of course Qxf5 wins no matter what black plays, but I told him to remain flexible because Qxf5 is not the best response to EVERY black move in that position.

it was an interesting position, unfortunately, my computer crashed later that night (stupid upgrading)