I used this - https://www.amazon.com/gp/product/B083J98ZFX
Get one of those 3.5mm splitters... hook it up to your recorder's headphone jack, then plug one end in your monitoring headphones, the other you plug one of those 3.5mm bluetooth transmitters in
https://www.amazon.com/Bluetooth-Transmitter-Receiver-Portable-Rechargeable/dp/B083J98ZFX
One simple solution:
https://www.amazon.com/Bluetooth-Transmitter-Receiver-Portable-Rechargeable/dp/B083J98ZFX
3.5mm audio jack to BT.
Yeah, something like this: https://www.amazon.com/dp/B083J98ZFX
This looks like it would do what you want, stream from aux cord to Bluetooth