So a short update on this story...

Before disassembly I air checked the clutches, and maybe I could hear some air leakage from the front clutch compared to the rear. Could also be that I "wanted" to hear
The transmission is now disassembled, did not find any major things, other than a really burnt front clutch of course. The seals on the piston was looking good, but 25 years old and maybe a little stiff. So I still hope/guess that was the root cause.
Rear clutch was fine, could still see the stamp B&M on the clutch plates so they were as good as new. Both bands were also fine.

I have had 5 plates in the front clutch but now included in the masterkit that I bought was 4. So I have to look for a thicker snap ring to decrease the clearance a little bit before assembling.
The endplay of the input shaft was also in the spec but high end, so will also try to find next thickness for the washer behind the pump.

But one question / concern
The low reverse servo piston is quite loose in the bore. Have not had any problems with it before, and I have had the stiffer spring from the B&M shiftkit in there. I could get a 0.019" when measured with feeler gauge...so is this something to be worried about or is it supposed to be quite loose ?