Max,
For regular page views we split it into two requests: one for lead section and another for the remaining sections.
As a general rule of thumb we want to put only the information that is needed to display above the fold (section 0, lead image URL, Wikidata description, ...), plus maybe some minor metadata fields that help us with housekeeping.
The gallery data for should go into the second request.
As far as housekeeping goes, I think it would be great to have the revision ID and some hash for the things outside the pure pagedata (gallery items, ...)
so we could determine after the first request if we could skip the second request because we already have that as a saved page or in a cache.
Ideally, those would go into an ETag, and the client uses an If-None-Match header the next time we request it, so that in the case we already have the data there wouldn't be a payload in the response, just the header.
Bernd