I would double-check that the sprockets are aligned properly and the tension is loose enough to not bind up. It might just bind a little until it gets heat in it. Is it an o-ring chain? Doesn’t look like it but hard to tell.
My method for lubing the chain is to spray it in the inside of the chain, so that when it spins the lube flings outward into the rest of the chain. What are you using for lube?
I’d say that 2 - 2.5 is good, I’ve always use the method that when you put your finger under the upper part of the chain and pull up, you can see 2-3 more teeth that when the chain is resting, everyone I’ve talked to always says a little bit looser on the chain is better.
Went from a Rotax Max such as yours to a ROK VLR 100. The ROK has a 10 tooth front sprocket, instead of 12 like my Rotax, and higher RPM’s. I ended up finding a “frozen” chain when I’d come off the track after a 10 lap session in our Tucson heat – the quality synthetic lube I’d been using for years just disappeared under the new engine’s tight front sprocket radius and higher RPM’s. I was advised to try a D.I.D. 219 HTZ SDH chain (not an O-ring) with Motul Factory Line lube. Couldn’t believe the difference. Now when I come off the track after 15 - 20 laps hand spinning the rear tires up on the stand seems as if the rotation will last for 20 seconds – almost as long as spinning the fronts. My friends are all going the same route with equal success.