Bluetooth reverse tethering
Bluetooth reverse tethering for Android on Windows 7/8/10 is not possible out of the box. It might be if you have installed the Toshiba Bluetooth Stack (see below).

As of Windows 10 1809 the Bluetooth PAN network generated by Windows' own BT drivers technically is not a "full blown" network like wired LAN or WiFi. Thus it does not show up in the list of network adapters to use the shared internet connection.

For the same reason you cannot bridge your wired LAN ethernet adapter with the bluetooth PAN (Microsoft documentation says bridging needs two "highspeed" connections - which PAN obviously is not).

As some googling reveals Mac and Linux folks are better off because these OSes can do reverse tethering with bluetooth.

If you really desperately need to do it based on a windows machine your only choice is to set up a virtual machine with Linux as guest OS and do it from there. There is a how-to in German on this solution at PC-Welt.