He didn't go SSJBE even against SSJ Broly. Probably because SSJBE was an anime only transformation that Toyotaro adopted into his work, even if AT allowed it in the manga.
Gohan fully activated the potential unlock after Gotenks ran out of his SSJ3 Ki so it has to be when the ritual was still in progress. If I had to guess, the time he snapped at the elder Kaioshit
Seems weird Freeza developed such precise Ki control despite only having 4 months to train up to God tier from fucking Namek SSJ level lol. But since it's modern :troll and Super I won't bother questioning it.
Goku's first use of Sign was only momentary, he couldn't even stabilize it long enough to avoid Jiren's second hit. Since we know Grand Priest >> Whis > Merus > Galu > Moro both in power and UI proficiency, it stands to reason that Galu's increasing mastery of UI caused power bloat.