Without building a city or attaching to one, an outpost can build harbors. As phoneticians or Carthage you can build their respective harbor replacements the prior giving a significant amount of gold and the latter production at a cost that equates to your 1st district growing. As a result you can do this then merge the territory with a city for some districts on the cheap, or just keep them as they are where they'll grow pop fast produce more districts faster or yield a substantial amount of money.  

For reference by the time I left ancient era I was producing 250 dust from 4 coastal outposts while I had 3 cities making food and building monuments.

I suppose this may not be a problem vs humans or aggressive AI, given that the 2 cultures lack a strong land unit but it feels perhaps unintended that districts can be added to a city on the cheap.

I liked this possibility, to build a harbour with outposts very much.

When attaching the territory, maybe the influence cost should grow with the districts and improvement in this territory.

Secondly, when attaching to the city, you are confronted with a stability drop with all this new districts. As it is right now, you can deal with stability easily. With a better balanced system in future this could be less exploitable, and a nice thing to do for outposts.