Is there a reason you can’t NTRIP directly from the R750 or SPS855? If you could do it that would be the best option.
Some related details in a recent discussion here: Set up own Trimble RTK base for Emlid rovers - Reach RS3 / Hardware integration - Emlid Community Forum
Your challenges start with the CMR+ if UHF (need RTCM), the 900mhz is different to the LORA and not compatible, and I don’t think you can relay/share from the RS3. I would actually like to be able to do that myself.