I'm not sure Toriyama would have been the one to decide to use this "technique". That seems more like the showrunner's call, or the script writer, or the storyboard artist, right?
I may be wrong, but from the beginning, I've been under the impression that Toriyama has had close involvement in the writing and scripting process. To an even greater extent than Super, you can tell all the gags, lore and quirky scenes are coming ungarnished from him. Of course, there are other writers and directors involved here, but most of the key moments seem to come from the big man himself. My hunch comes from how uncannily similar in tone this flashback feels to other pieces of Toriyama narration in recent projects:
When recapping the Red Ribbon Army arc onwards, the narrator is all like "Don't suppose y'all remember those Red Ribbon guys? Well, here's what happened..."
When explaining the Third Eye, the narrator's all like "Woah, hold the phone, let me just explain this Third Eye thing while I still can. So, there was this guy named Dabura..."
I'd be very surprised if Toriyama didn't have a large hand in this whole diatribe and the decision to place it at this specific point in the story.
Yeah, it being too exhausting for them to use Super Saiyan forms never worked as an explanation for me. Not a good one, at least. Like you said, it really shouldn't be exhausting (especially after the Cell arc). And like all of the post-Boo stories, Super Saiyan 3 seemingly doesn't prove exceptionally taxing on living bodies. But even if neither of these were the case....they have the senzu bugs! Vegeta needed to use (read: waste) one because he didn't transform. Meanwhile, Goku (who did transform) seems to have not needed one at all?
Indeed, if Goku and 'Geets have a problem with Super Saiyan in their mini bodies, it simply hasn't been established at all in the series.
You know, I've been meaning to make a post, maybe in the "non-thread-worthy" thread, about the most pointless senzu uses in the franchise. This must rank up there with both Goku and Vegeta taking them before fusing in in the Future Trunks arc. I never saw the point of that, like shouldn't the fusion basically restore their stamina anyway, at least to a level where they can fight Zamasu evenly? Oh, and Yajirobe stuffing himself with them and inflating like a balloon.