Thanks - that's a lot of good info :)
Sparkfun page suggests one can do some of these adjustments via software too:
https://learn.sparkfun.com/tutorials/what-is-gps-rtk/all
>1 cm accuracy is also possible with a few lower cost receivers (such as the NEO-M8T) by capturing raw streams from the GPS satellites and then post processing the logs with an open source program called RTKLIB.
Post-processing is wild. Lots of detail here: https://rtkexplorer.com/
There are even cheaper raw-data receivers, this one is popular among Galmon stations: https://www.aliexpress.us/item/2251832630341954.html