I currently shoot with a full frame Canon 5D mark IV DSLR and while it will be a while before I switch to mirrorless, I'd like to better understand what switching to mirrorless would entail. I've read a number of articles and posts on the switch to mirrorless but still have some confusion on gear crossover. 

I'm mostly curious about how much of my existing gear from my nauticam 5DmIV setup including EF lenses/ports/extensions rings could be used with a new nauticam mirrorless housing. Comparing nauticam ports charts between the Canon N120 and Canon 120RF port charts, it appears there is a lot of crossover and I assume my existing N120 ports would work although entesion rings may change.

I know you can purchase EF-->RF adapters which would allow me to use my existing EF lenses on a RF mirroless camera, but my understanding is those adpaters make the lenses longer compared to a typical RF lens, so I'm confused as to if/how that fits into a nauticam port since lens size, if the extension listed in the port chart would be the same one that's listed for the RF lens, and more importantly if using an EF lens with and EF-->RF adapter would decreases image quality. If the obvious option is to go with all RF lenses please let me know.

Definitely buy yourself the EF-RF Adapter from Canon. There is so much you want to transfer into your new system! For example the Canon EF8-15 a very important valuable lens.

You may even easily cross over from Nauticam to Marelux, as they offer Nauticam port adapters.

In the Nauticam system it's easy as the camera is positioned in the housing so that you can use all or you current lenses, ports, extension tubes and zoom gears on your new housing. Put it another way the when you use the EF-RF converter the RF port chart has a line item which directs you to use the EF port chart. It does this by positioning the lens flange further back in the housing by the thickness of the EF-RF adapter. That way the the zoom gear will line up with the control gear in your new housing.

You would be able to do this with other systems, but would need to buy various extension rings, zoom gears and sometimes ports. For example with Isotta you can convert Nauticam ports to work on Isotta by changing the lug ring, but not extension tubes and the zoom gears won't match. WHen the time comes prepare a spreadsheet to add up all the costs to see what you are up for in total.