There is nothing especially magic about circulator design.. you just have to understand what it going on.
I have desiged many waveguide isolators, mostly Faraday rotation type but also Y junction, with low loss in the 100GHz region.
Yes, machining ferrite is nasty (but not impossible.. it grinds easily and you do not necessarily need diamond tools)
No, you do not need to tune the magnetic field. If the design is done correctly, and optimised for the chosen ferrite magnetic saturation, you simply need enough field to saturate the ferrite, and it is then also not sensitive to external fields.
A well equipped micro machine shop is all you need, and a good designer