Can't speak to the brush issue directly...but

Speaking of astronomer/photographer... Back in the '70s, as a student at university, I used "swab" development on 4x5 glass plates (astronomical emulsions from Kodak).

A reasonable amount of developer in an oversize tray and a cotton puff (cotton ball, cotton batting - from pharmacy) that was gently, methodically, and continuously moved over the plate's surface (side to side, up and down, not TOO much pressure - it was to keep the fluid in motion, not scrub the plate) produced a very even development compared to other techniques (no nitrogen-burst development available, just trays - a small department). If I recall correctly, there was also an open-weave rubber mat under the plate to keep it from sliding about and to raise it off the bottom slightly.

As part of a project, evenness of development was checked by uniformly flashing plates to a moderate density then scanning with a scanning microdensitometer (a large, office filling beast that output as a trace on paper or punched a paper tape). Only the swab development produced a flat scan (indicating even density) across the plate's width and height. All other methods produced unevenness especially near the edges. (developers were MWP-1 and MWP-2).