Basically if you have replaced the motor mounts and inspected/replaced the transmission mount - your best approach is to plan on pulling the transmission and clutch. It could be any number of things internally to the clutch - glazed disc/flywheel from excessive slippage, contaminated disc from
oil, warped, heat checked flywheel, worn pilot bushing, and on and on. I would tear it down and then don't just throw a new pressure plate and disc in it and button it up. Have the flywheel surfaced or if showing heat cracks then replace it. Replace the pilot bushing/bearing. Inspect the transmission front bearing retainer and replace if worn. Inspect the throw out fork for wear and replace as needed. Inspect the back of the block for
oil leaks. Inspect the transmission input shaft tip for abnormal wear. Make sure the front bearing still turns freely without coarseness. Trying to chase a chattering clutch one part at a time is very frustrating and time consuming (I know). Best to tackle everything in one effort and replace or machine everything.