The fact of the matter is that actually beating endurance is practically an impossibility. There are, I believe, 512 waves total, and the HP increases exponentially at a rate that is quicker than your damage will increase. And on top of that, the lag will be so awful with all of those monsters with a such a high hp and dozens of gems firing off hundreds of times a second you can literally count the frames.
But there are certainly a few strategies you can employ to get pretty far(though, being a mid-game player you're somewhat limited by your skills). Many of them can be found in this forum, written by people with more experience than me. I'll go ahead and answer your specific questions, though.
Are prismatic gems worth it compared to bi-colored gems around wave 50ish?
Perhaps. It depends on the level and your mana farms. Keep in mind that prismatics aren't really worth it below grade 11 and as a midgame player your skills probably aren't leveled enough to make them very effective.
Can you give any tips on when/how often to manna pool, for example "x1 time per wave" ?
This is something you'll have to do largely by feel. Once you get a good mana farm set up, you'll be doing it far more than once per wave, but it's just something you'll have to learn.
Are many lower grade gems better than a few higher grade gems around this wave?
A few high level gems is better than a lot of low level gems at that stage of the game.
Are level 7 and above grade gems worth it, damage and price wise?
From what I've read, grade 11 is when it stops being worth it to combine gems of the same grade. Though with prismatics I believe it's around grade 13.
I notice that when I replay an endurance level, my exp doesn't go up?
You need to beat your previous endurance score for that level.
Any other tips you can give for mid(?)-game endurance levels?
Anger, anger, anger. You need to anger waves starting almost immediately to start raking in the mana. Speaking of raking in the mana, make Red-Orange gem traps to farm mana. These two things are vital to getting far in endurance.
Patience: Be prepared to spend days on a single endurance run if you get far enough. Yes, the lag gets that bad.