I brush develop my 12X20 negs in 16X20 trays. I started out with 20X24 trays that I already owned but the extra chemistry became a consideration. Brush developing gives me the even development that I want.
I use a Nuarc vacuum frame. I thought of using a conventional spring tension frame but Michael Smith, Sandy King, and others told me that the difference in sharpness between a spring tension frame and a vacuum frame is noticeable when one works with big film. I took their guidance on this matter.
Sounds like you're considering the move into ULF. Good luck.