Sometimes I have an urge to masturbate/watch porn, and I feel like I should pick up a Siddur and say Tehillim instead - as that's the best thing I can do to a feel a sense of deep Tohar at a time when I feel like I'm lacking it. Any suggestions of Tehillim to say in that moment? I could of course just start in the first Kapitl, but would be more interested to have a set list to turn to.

How about Tehillim 51, about Dovid Hamelech's regret after the story with Bas-Sheva?

If Tehillim works for you I would suggest saying general chapters that shift your focus away from P&M to something else unrelated to it. For me, Tehillim and davening didn't work because it wasn't enough to shift my focus from M and I ended up falling. You can also use the Distraction tool to find something else that is enjoyable and requires your full attention. This worked better for me than davening.

Maybe you could try a different approach and say those Mizmorim that express thanks to Hashem, like 100 or from 113 to 118, those that conform Hallel, etc. Just focus in thanking Hashem for everything in ur life, specially these Nisyonot that are hand picked just for your growth.