The role of the Hub is to point to resources, in contrast to pages being tutorials. It should be as complete as possible. If it's not digestable then it needs some restructuring. So yeah it might get even more stuff but I'd hope that we can avoid that by removing some (likely existing) duplication.
IMO "done enough" with sorting out content (which also required edits on mw:Developer_hub, mw:Manual:Database_access, mw:Manual:Special_pages, mw:List_of_simple_extensions, mw:Annoying_little_bugs).
mw:How_to_become_a_MediaWiki_hacker now really focuses on MW core and MW extensions, as (surprise!) implied by its name, but...
Still want to move the now totally generic (not MW specific) "Suggested Reading" section to a better place as it affects all and any development on all and any projects, but wondering where. (After that, update the intro section on mw:Annoying_little_bugs accordingly which is also a mess of any and every project and not MW only.)
The actual task topic is solved, hence resolving the task.
Remaining and welcoming input:
T181013: Consider synchronizing "Overview" sections on mw:Developer_hub and mw:How_to_become_a_MediaWiki_hacker and T181014: Consider having the mostly generic "Suggested reading" section of mw:How_to_become_a_MediaWiki_hacker on mw:New_Developers