Are you sure your Nikon scanner doesn't support ICE? I bought the LS-1000 in 1997 and it didn't have ICE, but the LS-2000 (and newer) that came out a couple of years later have ICE.
The older Coolscan scanners have SCSI interface which can be difficult to find now.
Note that ICE doesn't work properly with old B&W film or Kodachrome (at least with the Nikon software) but is IMO extremely useful for color negative film and other types of slides. Anyway, if you have the "wrong" type of film then ICE won't help, and it would be pointless to put effort into getting it.
Both a Coolscan and your camera will give better sharpness and detail than the Epson flatbed, but the Epson is probably the easiest way to get ICE. The problem with the 550 and other flatbed scanners is that the "real resolution" is less than 2000dpi (some, like the V850 do somewhat better) so a full 24x36 scan would absolutely best case be around 6MP "of information" and most likely less.
That said - how much info is it in the film? If it's taken with high quality camera with an excellent lens and on good quality film then it's considerably more. But it it's taken with a basic P&S or even SLR with a cheap 50mm then maybe a scan with 4-5MP (again real resolution) is enough to get most of the data from the film.
A Coolscan, even a LS-1000 give 2700 dpi and will significantly outperform the Epson 550, but with no ICE I don't see the point in trying to get it to run if you already have a R5 II and a RF 100mm macro. A LS-1000 would at most give around a 10MP scan while the 45MP sensor in the R5 is give way more detail (even if its' only a better picture of the grain). Assuming the lens and the demosaic-interpolation reduce the real resolution of the camera you'd still have a lot more detail than a 10MP scan.
With the R5 setup you'd be sure that you get absolutely all info from a 35mm film - at least form all standard types of film.
There are lots of option for camera scanning. The simplest is just a film holder, a good quality light source and something to hold your camera, but for 35mm film only you could check out the Valoi Easy 35 which is easy to use and should be compatible with your lens (better check though).