Can you quantify what you're seeing as too tight/too loose? FWIW, I'm getting good results with about 2mm of depression on the belt.
Re: The belt and small pulley. You know that I put a lot of study into mine, and I found that it really was important to my PecPREP graphs that the belt track down the center of the small pulley, and not pull to one side. The only explanation I can think of for this is that if the belt is rubbing on the pulley's keeper disc, then it must be fluttering a bit from the friction, and the belt teeth will be engaging the pulley at a bit of an angle and perhaps snapping as they seat? Can't be sure what the mechanics are, but I do know that my graphs cleaned up considerably when made sure that the belt tracked straight.
Re: Pulley wobble on the shaft, the ones I've seen are a good close fit, but I wondered about that as well. I tighten the two setscrews in turn, hoping to distribute the shaft clearance more, but I've no way of measuring whether it makes any difference - really just makes me feel better. Visual wobble of the pulley doesn't tell you anything, as all you can really see is the keeper disk, and it's a swedged-on part, and not integral.
Re: Your question about the post-600s periods... I don't see an 800s in mine, but I do have various other long ones past that, and they vary depending on whether guiding is on or off. Basically, I don't think they matter for tracking - if your mount is tuned well enough to handle the short-period waves, it should also take care of the long-period ones.
Mounts: ES PMC-8 EXOS2
Scopes: SV 102EDT, ZWO 60/280 Guide
Cameras: Nikon D5300, Altair GPCAM2-IMX224C
Software: ASCOM, CdC, AstroTortilla, BYN Pro, Sharpcap, PHD2
Computer: Thinkpad x230, Win7Pro/64