This is the best Bluetooth keyboard firmware. Check it out under https://ZMK.dev
If you are interested in Bluetooth dongle: check out build_folder.zip This can be flashed on a small Seeeduino with an nrf52840 soc (instead of buying one you could make your own (https://github.com/Nick-Munnich/mini-dongle)). It’s super handy to type on other peoples pc or in the bios when it’s not possible to connect to Bluetooth.