@Jeffrey Hutchinson yeah, this is one of those questions where the honest answer is "it depends, but here's what actually matters" — the 1.5m spec is open-sky CEP, so under heavy canopy you're realistically looking at 3-8m depending on tree density and how well the module handles multipath.
The 18Hz fix rate is actually what saves you here for trail mapping though. Because you're moving at walking pace, the high sample rate lets you do a lot of post-processing smoothing — when the signal gets noisy under canopy, you have enough data points to filter out the spikes and reconstruct a much cleaner path than a 1Hz or even 5Hz module would give you. That's genuinely useful for your use case in a way it wouldn't be for, say, a static survey application.
The multi-constellation support (GPS + GLONASS + Galileo + BeiDou) on the MultiNav Pro+ is the other thing working in your favor — more satellites in view means under a partial canopy it's grabbing signals from more angles, which helps a lot compared to GPS-only modules in the same conditions. Heavy old-growth forest with a closed canopy is still going to give you grief, that's just physics, but moderate trail cover with gaps should be pretty workable.
If you're serious about accuracy validation before committing, I'd honestly just get a unit and do a walk test on a known trail segment — compare against a reference track from something like a dedicated hiking GPS. The 18Hz data will tell you a lot about how it's handling the canopy transitions.
