Welcome to my world… 🙁 We chase creaks every week… on all types of FS bikes. Find the source… easily said not so easily done.
Have you tried: Rear Qr, cables outers, clamp on seatpost, casette on alloy freehub body, creaking against the grooves forced into the free hub body,Chainring bolts loose, worn pedal cleats, pedal bearings, etc etc etc
Oddly enough you would be surprised how many turn out be the seat clamp needs cleaning , greasing and refitting.
Does it creak when you are not sat on the saddle? say when you stand at the side and just press down on the top tube?
Also +1 the cracks in the frame!
zone