Hey ^.^ I'm also a full touchscreen player (I've been playing for a year now, rank 80k) my advice to you is change the way you position yourself when you play osu. There is always going to be vision block but what I do is sit on a flat surface (like a bed), put my knees in front of me facing upwards, balance the computer on my knees so the touchscreen is facing me, and place my not dominant hand on the keyboard so the computer doesn't tip and use the other hand to tap the notes. To prevent vision block, look at the screen from an angle so you see more of the screen and less of the finger. I just put the computer a little more to the left side to distort the view. As for double and triples, I would say that you just have to practice for doubles. For triples, I set my right click to the [I] key and just poke the screen and jab this key twice for 3 hits. This also works for streams.
By the way, some good songs I used to practice that don't have a lot of triples are 96Neko- Paintings? Oh, yeah. on Insane difficulty, Hantan- Airman ga Taosenai on Hard difficulty, and any of the Jump Training maps (they are unranked. there are 2 of them: Jump Training and Jump Training #2).
Paintings? Oh, Yeah. :
https://osu.ppy.sh/b/173015&m=0Airman ga Taosenai :
https://osu.ppy.sh/b/340104&m=0Jump Training :
https://osu.ppy.sh/s/110557Jump Training #2 :
https://osu.ppy.sh/s/163808I hope this helps ^.^ If you have any questions just ask
Oh, and by the way, don't listen to the people saying use a tablet. Touchscreen is way more fun... and I have both.