EtherRegen is a distant memory :-) Between Muon / ENO, my experience is limited to ENO which is nothing but all positive. The only reason ENO is no longer in my system because it doesn’t support 1Gb/s connection speed. I suggest you read up on thread by @grannyring which outlines his transition to Muon from ENO.
As far as M12 goes, yes it does not use RJ45 connectors, instead you need a M12 connector. If you go M12 route, you must buy either Telegartner M12 X-coded/MFP8 IE GOLD LAN or JCAT Signature LAN Cable GOLD M12 X cables. You need minimum of two such cables, one from router to M12 switch and 2nd one from M12 switch to Lumin X1. And 3rd one to connect Nucleus. For me, this was a end game solution. I am no longer looking to upgrade or tinker here n there. Pic 19 in my virtual system shows how M12 is being used in my system.
No, JCAT Isolator is not needed with M12. I bought one and sold it. It did next to nothing with M12 Switch.
Hope this helps!