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ExpandablePlanetMod

To directly address your scaling concerns, how to sensibly upgrade planets beyond 30 districts. The answer can simply be to increase the amount of "planetary mass" required for higher levels at an exponential rate.
Since this planetary mass would technically be a limited resource, dumping it all into one planet minelessly wouldn't be optimal outside of a hyper-tall empire build, and would still take some effort to achieve.

In addition to this, I'd follow a similar idea to the water planet size expansion, which is extracting value from other bodies in the game. Otherwise, we get planets that can grow themselves with their own materials. I'd suggest a "planetary mass" resource that can be acquired from destroying other planetary bodies, asteroids, etc. In a similar fashion to the [4.0] Stellar Manipulation Rewrite/Revival mod and its "stellar plasma".

The later stages should arguably take longer since the larger the planet, the more total mass it needs to expand its diameter/surface area. Plus, the larger the planet, the greater the specialisation potential it'll have.
Big planets are powerful, and should thus be hard to get. Otherwise we go down the power fantasy route.

You don't want fully upgrading a planet to get easier for the player, but instead have them keep considering if they really want to keep expanding to get better value later, or if they need to leave the planet at it current size to get value out now.

You could add tech to speed it up, and also split it into tiers, Such as the first version is obtainable after terraforming, which goes up to 30, the second version is after mega engineering or master of nature/world shaper ap, which goes up to 35 or 40, then have one locked behind having both of mega engineering and master of nature/world shaper, which goes even higher.

@ The Wrench: i am thinking about extending this mod to give more possibilitys for upgrading planets. i will think about some sensable way to upgrade planets beyond 30 districts.

Update:
- Tech "is rare = yes" | this was causing issues in testing. tech is now regular tech and can be researched by all empires.
- Updated Weights and AI Settings. AI should be able to research and build the building. needs further and extensive testing though.
- Update is compatible with all regular 4.0.X Savegames

Is there another version of this mod where the upper limit is customizable? I would love to be able to make size 50 worlds.

glad you fixed that bug, and hoping eventually itll work with AI too, nonetheless amazing stuff

Update: Mod works as intended now. planets will grow and the -20% penalty apply's as it should. Modified the building cost in order to get AI to use it but to no avail. AI-Empires can reasearch it but wont use the tech. I will try to fix this in a futur update. For now have fun building you planets to max size in the endgame and have fun with it :)

@Hellbrink thank you for the feedback. i will try my best fixing this issue