Hi Marcin,
Something like the AssembleTheBridge RPC sounds important enough to be reliable to me, or better yet, perhaps based on a replicated property. The main benefit of using a property for it would be that if you support join-in-progress, late joiners would automatically receive the most up-to-date state.
Unless you’re really trying to optimize aggressively for bandwidth I wouldn’t worry too much about using reliable RPCs. If you really need to you might be able to split your functionality into multiple actors with different NetFrequency values - but this might add more complexity than value.
Also, if you increase the NetFrequency on some of the actors with time-sensitive RPCs, you could try lowering the frequency on other actors that may be less important to compensate and even out performance.