I suck at runs too lol! What usually helps me improve them is to play them at different rhythms. Hopefully this will make sense. For example we have a scale:
1: ABCDEFG 2: ABCDEFG 3:ABCDEFG
1, 2, and 3 are all in the same run. By playing in different rhythms I mean starting out the run in say eigth notes in moderato for the 1st part, 2nd part in triplets, 3rd part in sixteeths, and the descend either in the same pattern or in a different one. It's up to you. In quintuplets, 32th notes..whatever. This is something I use to break the monotony of practicing scales, makes it a little bit more fun and interesting, and it keeps the brain awake (so you won't end up just mindlessly drilling them). Also try playing the notes like this as well: AA, BB, CC, DD, EE, FF and so on. Again, these are just tips that help me out.
In regards to reducing tension, I'm not an expert so I can't give you any -qualified- advice. But my personal experience is, back when I started out i used to play a steady wrist, very static, and I moved with my hands (the hands basically did all the work.) Wasn't a big deal when I played slow works, but when it came to mozart or other pieces, the tension used to build up on the wrist quite quickly. What I have learnt from friends, my teacher, and people on this site is to keep a relaxed wrist and to use your arms, not hands, to guide your movement. If you're already doing this, ignore me, but if you're not try it and see if it works. But start slowly and then gradually build up your speed.
And rhythm? Can't help you there. I still can't count properly lol, so when it comes to poly-rhythms I do it by ear. No one has called me out on it yet!
Anyways in regards to reading, I feel that it greatly helps in learning pieces, especially the harder ones. Advanced and later [mid-romantic to 20th century] music is harder to pick up by ear because they're more tonally/harmonically ambiguous. So I don't really have to kick myself when i decide to learn a scriabin piece or some weird late mazurka by Chopin - reading the music helps a ton! But then again there are people that can pick up individual notes from bloody clusters XD, so it might not apply to them. Plus if you ever really get into piano, you'll find yourself digging up obscure music, and unfortunately a lot of them don't have any recordings yet. So it'll definitely help you out in that area too. It's easy and quick to learn, you won't have any troubles with it So I recommend you do it!