So. This could still be hardware even though diagnostics check OK. Those diagnostics generally check for presence, but not functionality. So the iPad can know it’s there without it being functional
And even though the wifi chip and the bluetooth chip are the same, they have separate lines of communication to the necessary board components. So If you’ve reinstalled the software and it’s still not working I would say it’s a really good bet it’s the hardware. The only other thing I might try if you haven’t yet is to restore it to factory, and don’t reinstall any of your user data, so no icloud login and no restore from backup. Just a clean slate and see if it works.
Otherwise, your best bet to get it fixed is to take it to somewhere that does repair these that isn’t Apple since they will only offer out of warranty replacement for the whole iPad. I’m not specifically familiar with what could be at fault on these, if there’s a bluetooth specific antenna, but it could also be something on the boardthat effects just the bluetooth functionality.