Sorry if I’m not very good at writing rules, but I’d guess after the button is pressed you create a timer that upon expiry raises the brightness a bit and then re fires the timer and is killed upon receiving button release. There are some threads here that talk about timers that work like this, but I’ve no idea where. Maybe this would help:
https://community.openhab.org/t/design-pattern-expire-binding-based-timers/32634
Ira