Any nearby ground should work. Make sure it’s actually ground/B- though and not switched in any way.
Technically there’s no such thing as a monopole, it’s a physics impossibility. There’s always a second “ground” element, it’s just a case of whether it’s tuned or not.
The single wire antennae are dipoles where the second element is the B- plane in the PCB.
The frsky antennae are dipoles where the last ~50-15mm of the insulation sleeve acts as the second element.
The big AM radio towers use the tower itself and/or the Earth as the second element.
Car CB antennas use the roof (trunk lid, etc) as the second element.
The frsky version self-tunes to an extent as the length of the coax itself is tuned, but it’s not as good at resonating as an actual 31mm element would be.
The whoop style B- plane is a mess as a second element, but it’s there.
It’s just like you can’t have a voltage without having a return path for that voltage, it’s all relative.
Some coax done frsky style to get the antenna out from between the PCB and the lipo would definitely help range. You could also desolder the antenna and solder it back onto the flip side of the board.