Tried and tried to get these to sit tubelessly on bontrager rhythm rims but just can’t. Tried soapy water but just no joy! Any other suggestions?

Have you inflated them with a tube first? If not do that, leave it overnight, the tyres will stretch a bit. When you’re ready to switch to tubeless leave one side of the tyre seated in situ, whack the tube out the other, and the new valve in, and inflate.

had the same issue with them using a BOR 388 rim.

Turn the tyre in-side-out over night and have another go tomorrow. the tyre opens up and sits against the rim better.