Maybe I should just wait longer for answers, but you do not do justice to me. This is not a map, but a concept.
It also works on large maps: you just need ways to grow a planet faster than you claim new planets. I did not say, that this is limited to map borders and few planets.
While the discussion is stuck for months (which is not a problem) and giving only the devs everything to do and watching the time passing by, there are ways to solve the topic, even if not in the exact way, because limitations in expectations to a solution may give you a better result sooner or later, but you have to wait for longer.
Instead of waiting for 'real features', this is a different approach that can be welcome for a community.
Shall I tell you a secret?
Good Ideas grow slow.
And players who are willing to help, either.
The concepts has many aspects. The present can be unfolded in a way that everybody can like it. If you throw it away, it is not my problem.
So my Idea of a tall empire is to boost the speed and the power, single planets can get, with every possibility that can help here, for example more buildings/improvements that make you powerful on your island (and inclusde every ever discussed content, you might add it here)
One way to improve this aspect of the game is to design the right maps, that support this playstyle.
You boost the growth of the planets enough (with map tools here) to make tall developements attractive!
You boost the power and the production, relative to the amount of planets in a galaxy.
Quality instead of Quantity, or Quality being equally strong to quantity as on the map above.
I dont see why this is an other topic than that which 'you' talk about.
Alternatively you can also limit the wideness as you adressed, yes that is also a way. Each way is good.
Now, we do not limit symmetry either, but coming from multiplayer-C&C, symmetry is always a huge fairness-factor, which becomes important if you go away from the random race for planets with 'in the complete result fair' conditions, because the faster player wins, which justifies the larger empires somebody gets.
In tactical matches, enough people love them, symmetry is a must-have.
In a galaxy game of course you want to explore. But that is not a factor, symmetry is not required, you just need to change the 'ratio' of wide against tall, not the size of galaxy is important.
Now, isn't that a start to develope together?