Solved.
Well, if you're suspecting calibration issues, you could try slightly adjusting the spindle position and/or speed of the motor until the disk could be properly dumped?
You have three adjustments: spindle alignment, read head position, and motor speed.
the guide for correctly aligning the spindle is here:
https://www.famicomdisksystem.com/tutorials/fds-repair-mod/belt-replacement-adjustment/
If you have a scope, you can use it to set the read head at a position where the signal is strongest using this guide:
https://wiki.console5.com/wiki/Famicom_Disk_System#Alignment_with_Oscilloscope
you can also try doing this with a good digital caliper.
motor speed can be tuned with the copy master program:
https://www.fdsstick.com/fds-copy-master-v1-1-lastest/
I believe a speed of 4-5 should be optimal when a disk is inserted.
good luck. if you want to sell it afterwards, let me know.
Thanks for the links, fingers crossed everything goes smoothly!
Will update if I can get anything working.