I think you are overthinking it.
In my experience with my RS+ units, is that they are more precise than my ability to level a rover-pole on a bi-pod or taking into account the possible flexing of the GCP plate.
With a recent experiment I compared the baseline of the Reach RS+ combo with a total station edm measurement. They agreed to within under 1 cm at 680 meters: Reach RS+ baseline vs total station EDM
A few days ago I flew 2 drones (Phantom 4 Pro and M600 Pro with a Phase One iXM 100 mp) over a 436x176 meter area, using 15 GCP’s. Combined 3D RMS reported by Photoscan was ~1.6 cm for both projects.
As long as you got decent sky view (doesn’t have to be excellent as such), you are in good shape.
So, relative precision is not an issue.
The absolute precision is again just up to either your baseline to a NTRIP station, or your own ability to mechanically line up your own base over a known point, given the known coordinates are still valid (which is probably much more likely to be a significant error than RS+ precision).
I am also not a land surveyor, and I haven’t tried any L2/L5 gear, but cost vs performance, is, as you can see from the numbers above, unbeatable by a 30k usd setup. In decent sky view, their performance will be neck-to-neck I am sure. Only in limited sky view or noisy environments the multi-band solution will have an advantage.
It too will have limitations in limited sky view, where a total station simply is a better option (hence they are still made for and used outside