Most of the T-mount adapters for mirrorless cameras are deep, designed to make the mirrorless camera mount-sensor separation the same as old DSLRs, much longer.
I think the new full-frame Nikon mirrorless Z-cameras have a mount flange to sensor depth of 18mm or so. This compares with 49mm for the average sensor to mount distance. What I've found is that in-travel in focusers doesn't leave much room with these long T-mounts used with mirrorless cameras. What I would like (even though in some instances I'd have to use an extension tube, depending on the scope) would be a low-profile T-mount system like you have with the DSLRs. Here is a comparison of the T-mount for a m4/3rds mirrorless camera and a Nikon DSLR system: