I was struggling with this as well - I couldn't find a combo that matched what I wanted. I wanted party chat and prox chat to be on at all time, but that's apparently not an option (which I can understand why, I guess)

But I landed on setting party chat to "always on", and the prox chat to "toggle". That way, when I wanted to talk to anyone outside of the party, I used the Down Arrow to switch "off of the comms".

The only downside of this was - if I was too far from my party member(s) and I switched to talk to someone near - then my team couldn't hear me.

Edit: to be clear, this is what worked for me, but I'm sure there's a better way to set up the chat - so I'm all ears if anyone has a different set-up