So far what I have understood is that there are different paramaters that are linked and that can play a role in a MAVLink RTK GPS. I only used the GPS_INPUT MAVLink messages.
First, the GPS Fix is only used when the drone chooses its best GPS. That means that if you have a really bad GPS but you say you have a high fix, the drone will only consider your GPS.
Also, it uses the data of only one GPS at the time. So if you have a GPS that provides only really accurate position and no velocity, the drone will do with no velocity, even if the other GPS provides velocity.
In the GPS input message, you can use flags to say which information you provide (only position, only velocity or both). You can also give the estimation of the accuracy of the different vertical and horizontal velocity and position. Note though, that the drone will use as minimum accuracy the accuracy provided in the full parameters list under the parameters EK2_VELNE_M_NSE, EK2_POSNE_M_NSE, EK2_VELD_M_NSE, EK2_POSD_M_NSE.
Anyway, using the GPS_INPUT message you should be able to provide accurate GPS information to the drone. Then starts the other work of proprely calibrating the PID and other parameters...
I hope it gives you some answers, it's not really clear but I'll try to answer more specefic questions
Good luck with your projects,