For the Ublox F9P, it will hold a baseline for 100km. The stated accuracy of 10mm+1mm/km for the first 20km is stated in their manual, but will hold a fix far outside this.
If you do not need the extreme sub cm accuracy, using only one cellular base could be a very easy option.
For pass to pass accuracy you also do not need absolute accuracy of your base coordinates, whatever the base surveys in is fine as long as you write it down and use the same base coordinates for the field you are working in year after year. If you are not using tramlines or trying to run the same sprayer tracks and start fresh every time, just surveying in new at location every time is fine.
Lora radio, or other data radio types are also very useful. But usually require line of sight to work, which will involve setting up a tower at the base. This is my favourite option, no bill, fairly long distance, ability to use custom refresh rates.
Alternately you can setup Base Monuments at different fields and bring the base with you connected locally through radio.
For high performance RTK applications nothing is better than radio, Transmission is almost instantaneous, you are not limited by data charges for high correction refresh rates and using all GNSS constellations.
Minimum correction data is fine 1Hz for each constellation used, 0.1hz for ARP coordinates for 99% of farm work. High refresh corrections at 10Hz arp still at 0.1hz, are only really beneficial for high accuracy demands like dozer blade control, and laying tile. But they are fun to play with.
According to F9P manual corrections should ideally be sent to the rover within 50ms. So having to make a hike to an NTRIP server on the internet and back usually has a penalty of 60-300ms. NTRIP is a very convenient way to handle corrections but is slow. Will it affect most farming operations running stale corrections? certainly not, so ntrip is fine.
The biggest question to ask when setting up a new base is, “What is the maximum amount of error i am willing to put up with?”
Buying commercial corrections for RTK, in Alberta for example, you may get stuck using a base station in the nearest town over +50km away or more, with no VRS. Trimble equipment currently can hold an infinite baseline, but not a very accurate one at extreme distance. But its still full price, lol.