Sounds like I should elbow the filterwheel idea as expensive overkill and to avoid back travel focuser problems because I will want a good 0.5 reducer on whichever scope I choose in the end.
I would not say it is expensive - but they do have some associated issues. Really, just a matter of prioritization.
Another avenue you may want to explore are filter drawers. Almost the same convenience as a wheel with less bulk and potentially less back focus issues:
GeezerGazer uses something like these with his 300mm telephoto lens. Might actually make more sense than the usual telephoto adapters, I need to look closer at that myself. That whole Prioritization thing
Just my $.02
While you’re getting the 2” .5x reducer you might as well get the .7x as well. I use both, though I use the .7x more. IMO, 3nm is pretty narrow. Depending upon the night, I actually tend to use my 12nm more often than my 6nm, but I’ve never wished for any narrower than the 6nm. FWIW, both of mine are Astronomik filters.
Yes, the 2" 0.7x reducer is nice to have. Hard to wrong for only $50. Especially if you use the device with Newtonians, which are challenged for back focus distance.
I took some approximate measurements on my 16" Newtonian and an AstroTech 60ED refractor and they were consistent. 12-15mm back focus to use a 0.7x reducer, 55-58mm to use a 0.5x reducer (on the refractor, the Newtonian could not do it).
With regards to filter choices, lots of great advice here. In the end your optimum choice will be a mix of aperture, speed, target, and personal preference. When you use a filter that is "lighter" (specifically, the 12nm) it is not easy to know if you have "optimum" filtration. On the other hand, when you use too "heavy" a filter, you will know it right away!