Two considerations to weigh:
The 18mm spec seems not be a problem for the Mod3C (others are already using if for that, and I think that is about the Mod3C sensor size - I calculate 18.85 mm), but I suppose that means it limits FOV with any EP with a field stop larger than that for visual use.
It's very difficult to predict where the hard stop to the FOV will be without knowing everything about the corrector/reducer, as well as the eyepeice behind it. There will be vignetting for sure when the field stop is 46mm, and quite possibly a hard stop stopwhere. Without specifics, which are likely not availble anywhere, that's as close as one can get. For prime, 18mm isn't bad at all. The 13 Ethos' effective field stop is spec'ed at only 22.3mm. That's not the actual field stop size, because there are field lenses before it. Again, it's a matter of specifics that you may not find anywhere. For visual, it's not likely not going to be terribly noticable with the available eyepiece options (see below), but with afocal with the 67PP, wth its ~46mm field stop, something has to give. How much is the mystery.
I can usefully use EPs with field stops up to about 28mm with fast scopes like this. My pupil has sadly declined from >7 mm in my youth to 5.2 mm now. I hope the decline has stopped here, but have no reason to actually believe that it has. This is an additional reason for my interest in NV which no longer requires wide pupils to see faint objects.
You have to consider the eyepiece's focal length as well then. If 5.2mm is your pupil, with an f/2.7 output to the eyepeice, that's a ~14mm (5.2*2.7) upper limit to eyepiece focal length without wasting some aperture. With the P2 as the visual corrector, it's about f/3.1, pushing it up to around 16mm eyepiece focal lengths. So that's fine for one of the 13-14mm 100 degree options, just shy of a 17 Ethos. A 17 Ethos isn't a terrible waste of aperture, just a little, but we are all getting older.
An f/3.7 mirror, with a P2 as the visual corrector would juice that up to f/4.2. That would allow a 21 Ethos! Whether that is a benefit at all depends on the relative effective focal lengths. For the same sized aperture, comparing a net f/4.2 to a net f/3.1, the added field (ratio of focal lengths, assuming same AFOV) needs to beat the ratio 4.2/3.1 (~1.35). For a 13 Ethos to a 21 Ethos, the 21 Ethos clearly wins (21/13 ~1.6). For a 17 to 21, it's a slight loss.(21/17 ~ 1.2).
There are all the other combinations of aperture, f/ratio, and CC used to consider.
Extension tubes for visual using a P2/HRCC are in your future if you build around the ASA, but so what? Those have tunable tops ready to go.