Automated boom refueling has already happened:
And the Navy's Boeing MQ-25 is LO and obviously a drone. I'm sure it does a lot more than just buddy tank too. Hell, the even Navy did automated receiver AR in 2015 with their X-47 off an Omega KC-707.
So, LO tankers aren't "that" hard. But scaling from fighter size vs transport size is definitely a challenge. It'll have to be a custom build like the B-21. Why not just build bigger fighters, I dunno...let's call them interceptors...and stuff them with enough gas to make it the fight and back (ie: MIG-25/31, F-14, J-20, etc). A lot easier than building LO tankers, IMO.