That speed will be a bit fast for a 6" cutter and anything larger will definitely way too fast.
The angle won't be a real problem other than the shape of the resultant glass. The grit going up the sides of the cutter will trim the excess glass away.
I might note that if you get 25mm an hour of cutting, you're going nice and fast.
The best thing to do is find a shaft and two bearings (even just a pair of blocks of maple with close fitting holes in them) and a pair of pulleys dropping the speed of the lathe will do you just fine. The cutting action is self guiding once it gets started.
FWIW, one of the guys that occasionally stops by the mirror making class that I'm running trepanned a 15" mirror by hand and, while he said that he's never going to do something like that again, he did a nice job but it took many hours of cranking the end of the shaft to get the cut done. He went through a 7/8" thick piece of glass. He did the pair of blocks on a piece of pipe, put a crank (90deg. elbow and some more pipe!) on it and lubed the blocks with some grease and had at it. He turned so that the threads would tighten as he went along. Very primitive but it worked!