My bike had been sitting for a few years before I rescued it. I know you said you flushed the clutch several times, but it's probably time to rebuild the clutch master and slave cylinders. I had flushed once, still had problems, so ordered the OEM rebuild kits (very inexpensive, minimal parts to replace) and did it. Problem solved. Check out the peanut butter I found in my slave cylinder picture.
a) Clutch Master: Part #3 and maybe 28, 29, for the master - and maybe 8 for the cup reservoir,
b) Clutch Slave: Part# 6, 9, 13, (and I replaced the spring #8),
c) any decent quality DOT4 brake fluid.
optional: Spiegler steel braided clutch line
btw, clutch slave cylinder parts are located in the *left crankcase cover* section of the OEM catalogs.
Don't forget new crushwashers, and I always bleed air at the master (via the banjo bolt) as well as the slave.
Very high chance your fpr diaphragm is toast. You could be having problems with it even if it passes the so-called leak test.
I'm betting you used proper wet clutch MC oil, but my bike HATES 20W-50. 10w-30 or 10w-40 full syn would be my recommendation.