IMHO Goku was entertaining Assault Mode Frost and it is only his final transformation that gave him trouble.
But it is only a small trouble; SSJ Goku can stop Frost on his tracks. SSB Goku cannot even do this to Golden Freeza. Yeah, three years of training, but still.
It also leaves us with the problem that Freeza, after fourth months of training discovered Golden mode, yet Frost, with a lifetime of actually fighting around planets (I don't think he trained four months just before the tourney? Freeza seemingly only ever rode his chair and blew up planets, so I assume Frost was more active than Freeza) somehow became stronger than RF Freeza, but did not discover Golden mode.
So the simpler explanation is that Frost is weaker than Freeza and that Goku is just humoring him so he can show off. Partially cause he thought Frost was a good guy anyway. On the other hand, DBS implies Freeza was holding back before going Golden too.
I don't make much of the 3 years of ROSAT cause, honestly, Goku and Vegeta are kind of shit at training on their own; Goku and the Earth lucked out that Gohan was there with tons of potential to channel into SSJ2, cause otherwise everyone else's training on ROSAT did not get them past Cell. Not even close. It took Vegeta other 7 years of gravity chamber to edge past Cell saga Gohan, and Goku discovered a transformation useless on earth. Good job guys.
OFC maybe they are reserving SSB if they are suspecting they are stronger than Beerus now but don't want to reveal their hand too soon. It's hard to tell. Before going into ROSAT, Vegeta suspected they were at the limit of their powers, but it is possible that Bulma warned Vegeta to keep their power level secret if they surpassed Beerus; Bulma is super suspicious of the whole Gods of Destructions business.
