Sectors are a little ehhhh, must say... the five planet thing feels too low, that's certainly true - recently I've been sending out fourish colony ships per year and I'm always handing over super underdeveloped locations for the AI to manage. Also been filling in gaps too recently since I uplifted a continental world preference species and those newly established colonies just end up going straight to the AI as they're in connecting areas or alongside another less recently colonised planets in the same system.
I think at least planets without the planetary administration building should not count to the cap, if nothing else. That way it gives some time for them to be developed before one is forced to hand them over. Sectors also shouldn't have control over space ports in my opinion - they never seem to do anything with them, from what I've seen, and things that you build there end up going automatically to them (even though you can control them weirdly) leading to them not appearing in the overview. A particular hassle with colony ships.
I'd much prefer to see a system modelled sort of like the old "distant overseas" system from EU. As in you have a certain range from your capital planet (which should be moveable mind, with restrictions) anything outside of that range is classed as being overseas and is penalised as a consequence. Sort of like the autonomy modifier. This range should be increasable by tech, probably alongside the new warp/wormhole/hyperdrive techs (better travel = better communication, better governance). Sectors should be a way of getting around this by giving territories in your outer zones to them, having their own range from their own capitals. How much they control their planets, however, is something I think should still be down to the player - as in boxes you can check, a little like the prevent redevelopment, "player builds" etc. Obviously would need some reworking of how they give you resources, but if they build I imagine it could be on a sort of "loan" like basis where they pay you less until they've paid for the building they've chosen to construct. They also still shouldn't have control over their spaceports. Their effect should be to provide more efficient administration of certain zones and allow the player to avoid micromanaging if they so wish, but not force it.
Consequences could be attached to these zones though, but that's a DLC thing I guess. Demands for autonomy, manifesting through say ability to set own policies that end up conflicting with central policy (and upsetting outsiders), keep more resources (reducing what they pay you), have own ethos/government and personality that may be different to yours (a bit like say if you had a sector that took the personality of "fanatical purifiers" they might get annoyed at how few wars you're having and/or failure to purge pops) etc. There's lots of room for things that can be done with the sector stuff, must say, but in its current form it feels a little frustrating.
Just my random thoughts
