Welcome!
What is the best way to maintain a stable link between the base and rover at distances up to 1/2 mile?
WiFi is best, then cellular (you’d need two phones, one for the base, the rover is usually your own phone). If neither of these is viable at your site then yes, a packet radio is the next best option to go 1/2 mile.
Since you’re just getting started, I’d recommend not focusing on the radios for now. Get your Rover/Base working at home with cellular and WiFi, and then move to the internal ESP-NOW radios. They work great up to about 500ft.
I’ve seen a lot of examples of people placing a base directly above a known point and setting a manual position, but using the rover to hit a known point to calibrate the system would be a much easier workflow because I can set the base in a safe location near the work area and no tape measure or alignment would be necessary.
Personally, I would drive a stake or marker at the site. Then capture 24 hours of RTCM, run a CSRS based PPP on the data. That will tell you the exact location of the marker. If altitudes are important, read up on Antenna Reference Points and be sure to subtract your pole length and ARP from your results to get marker’s accurate altitude. Then, store the known coordinates in the base (see Fixed Base mode). Every time you setup on site, put the base over the marker and turn it on. All readings from your rovers should align between days of capturing points. If you want to get really accurate then you need to take multiple days of PPP and compare, throw out any that look off, and average those within bounds.
Happy marking!