Bluetooth Not Available
If one reset of the SMC doesn't restore the function, you've done all the triage except for one last ditch option before hardware repair or dropping an USB BTLE adapter on.


Install a clean new 10.14 Mojave installation onto an external drive and let it update the firmware and then test without any user data migration or app installations. A total clean OS install can sometimes restore the drivers or state of the adapter if it's hung up and not a hardware issue.
Hardware repair is likely going to be costly so I've had very good luck with all manner of BTLE 4.0 dongles that use USB connectors. They cost $9 or so for good quality - try and look for one that says Mac compatible or buy from a company that accepts returns or can offer pre-sales assurance your chosen macOS will work. OWC is a good solid Mac centric vendor but I don't see them stocking, so this Amazon search should get you going: https://www.amazon.com/s/ref=nb_sb_ss_i_1_4?url=search-alias%3Daps&amp;field-keywords=btle+dongle