Or what about a used a9? I can continue using the a7R II for tripod shots with my manual-focus Loxia lenses, but could enjoy a better AF experience with the a9.
The thing I like about this scenario is that it keeps my still-works-fine a7R II relevant while giving me a much better handholding experience.
This is what I use, the A9 AF is great very fast and locks on, if you get a 200-600mm then the A9 is the best body to have with it, I have used it with a x1.4 TC. It will lock up at f11 or better.
I also use Loxia lens with the A7Rii, keeping this would allow you to get the x1.4TC with the money you save. Also think about the file size of the A7Riv.