The Ublox ZED-F9P receiver is capable of leveraging dual band across all 4 main GNSS systems to create its solution. It is very accurate. If you combine it with RTK (either local, SBAS, or NTRIP) you easily get sub 5cm accuracy in realtime with sub 30 startup times. (Longer if its a really cold start)

You should also have a look at the much better performing Septentrio Mosaic series of GNSS receiver boards. There are dev kits available for a dual antenna heading RTK receiver for around €750. And €645 for single antenna. Dizzyingly cheap compared to what this class of receivers cost just a few years back.

These support all GNSS constellations and signals and do a real 50 Hz PVT in RTK mode. And they even come with a built in, quite useful web interface.

I checked out the website. Its not clear to me why they are superior to the F9Ps.

Around 250 bucks for those wondering. Bit heavy for a hobby project but less than expected

We use some f9p's from Ardusimple for building an AgOpenGPS autosteer system.

I'm somewhat curious how one would determine the true position to mm accuracy so as to compare. Put another way, if something says you're at these coordinates, how do you conclude "it should be 1 mm to the north"?