Truthfully but understandably...yes, overthinking a little. ?
GMRS and 70cm ham bands are still UHF, and quite similar. There's no reason you couldn't do the mount of your choice, either mag or hard mount, and swap antennas at will. NMO mounts and NMO antennas are plentiful, and almost anything meant for UHF that covers the 450-470 is usable, whether or not it states GMRS. A SWR meter will be a very good thing to have, though more to double check than necessarily tuning. Most mounts, whether hard mount (bracket or drill through) or mag mount will come with cable; you'll find a few options on connectors, along with unterminated, but most radios will be uhf connector (pl-259 is the male side, so-239 is the female side). Not familiar with the swivel mount, so I'd be a tad wary about the antenna bouncing against something when folded, and might lean toward swapping to a shorter antenna off-trail. depending on your trails, having a short antenna available might not be a bad thing anyway (more trees and brush on the trails here).
i currently swap antennas on a midland mag mount; it's gone through a variety of antennas, both gmrs and ham. I mostly run a Browning uhf 5/8 over 5/8 wave which trades some gain for more frequency flexibility; it plays nice with both 70cm and GMRS. Since the Browning is a little tall at 33" above the roof of my truck, it gets changed out for one of two (70 cm or GMRS) Laird 1/4 waves as needed for clearance (which are only 6"-ish tall). radio wise, i previously used a mxt115 for GMRS, and various handhelds for ham, but recently put in a mobile that covers both (90/95a certified Vertex).
Any SWR meter that will handle the 462/467MHz range will get the job done. Mine measures SWR, power, and frequency. They're not that expensive.
https://smile.amazon.com/gp/product/B01D86IKIQ/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1
As for coax:
RG-58 is fine if I'm messing around in the shack. For a permanent install I use nothing less than LMR-240 (including my HF antenna). The VHF/UHF antenna runs around the shack are DRF-400 (same specs as LMR, but 20% cheaper). For a mobile install, you're going to need to balance your acceptable coax loss with what is practical. I'm using LMR-240 in the mobile install.
