It is difficult to offer a lot of substantive advice, as your opponent played a miserable, passive game. By the fifth move, you were considerably ahead in tempo. Black had just about zero activity. I like 8.Nb3 more than Nf3, as it strengthens your grip on the queenside, while your bishops can dominate the kingside. 9.a4 was fine, because sending your N to the d file just blocks up your bishop, and since you're so far ahead, you should get ready to attack. I would not call Nd5 pointless. It is a wonderful square and the N is firmly entrenched unless black decides to trade. I like Qf3 more than jumping the N around. Given your opponent's abysmal play up to that point, there's a good chance he wouldn't even bother avoiding the discovered check. Bg5 is another possibility. Keep developing if your opponent is going to allow you to do so. I would have kept the N at f6, then developed the bishop to g5. There is nothing black can do to kick them out now that his pawns are all forward and askew. Your question on the 20th move is pretty irrelevant... if the guy has to give up his Q to defend his K for a few more moves, the game is yours, and you're not yet at the level of worrying about having an aesthetically pleasing victory. Most players aren't. That's about all I can offer. It would be more fruitful to post one of your losses, so you can correct your mistakes in games against opponents who know what they're doing.
The Defeat of Victory
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I have never ever managed to get a good game. All those I won were where I had bad opponents and all those I lost was where I was winning but missed a mating net.
Anyway, here is the game.