Having been around both stationary circle saw mills and the "portable" band mills, maybe I can throw some things out and mudding the water a bit more...
The saw on a circle mill never shuts off. We powered ours with a PTO belt drive. Honestly worked pretty well. The log carriage moved with cables, a big lever was used to engage/disengage various belts which in turn pulled the log into or away from the saw. Not real accurate, but that is mostly from the alignment of various components. We could have improved it, but were ready to try something else, not exactly a safe working area either.
The bandmill used a 16 horse briggs to drive the wheel the band saw was on, and an electric dc motor via small chain to turn gears that were mounted to vertical all-thread in 4 corners to raise and lower the head. another dc motor pulled a chain and moved the saw thru the logs. I have seen saws with an electric winch to raise and lower the head - which honestly looks pretty simple. Pushing the saw thru the log is really not bad.
My conclusion would be to not use your tractor to run the saw. Buy a small gas motor mount it on the saw head, use belts to drive the saw and get the right RPM's. Get and electric 110v winch to lift the saw head, then manually push the saw. Use the tractor to move logs. If this sounds like too much work, find a used woodmiser, or even a small new one for under $8K. They hold their value pretty well, and you could stop designing something and get to working. Unless your retired or gainfully unemployed, your time is worth something right?
