While i understand the value of keeping things simple, I do think that "put money into the colony and get whatever you want" could use a few more steps.
Thoughts:
1. Having ships require resources, and not just cash, to build would help a lot. Want low tech ships? Cool, you just need ore. Want high tech? well you might need a source of transplutonic. This should extend to colony facilities as well. The basic star base shouldn't require more than cash, but the 2nd and 3rd tiers should require resources.
Now you can always import this as long as you're not at war with everyone (or maybe smuggle it if you are) but that would be factored into the price and make the ships much more expensive and difficult to produce (both for your personal use and for your colony). So it's still "put money in, get stuff" but if you want to spend the time to figure out resources, you can save a lot of money and get stuff faster. It could also lead to caring more about trade outside of the numbers, or gearing up a trading fleet solely to go scrape all the organics or transplutonics or whatever you can find so you can haul it all back and setup a new industry or build a specific ship.
2a. Further I think there should be more thought to really making a mini faction, not just a colony. Maybe this is already there and i'm not getting it, but I like the idea of having a world that's solely just there to feed the rest. Some super bountiful land that's maybe costing me money to run, but is feeding my other 4 planets. This then lets me justify setting up on some hellhole to mine it for all it's precious ores. I get that a full blown 4x is not the goal here, but allowing that sort of functionality adds a lot of interesting gameplay. You can asses planets differently, but you can also have enemies disrupt your trade routes or cripple your empire from hitting one super important planet.
2b. To expand, outposts for resources might be nice. Limit them to only working in sector, but you've literally already got mining and research stations in the game, so why not allow the player those? Let me harvest the gas giant for something interesting so it's more than just an expensive place to survey, and doesn't require a full blown colony to manage. Think of it as the tiles adjacent to a city in civ, which you then upgrade with workers. I should really be looking at the entire system more than just the planet, and right now I don't feel that. These should probably be very costly to setup/maintain as well, so it's more "i can trade my income for X resource if I have Y feature in the system" vs "Spend more money for more"
3. Tech levels should probably exist in some sort. I was hoping there'd be more blueprints for colony tech. Possibly some [REDACTED] tech that makes settling various planet types easier. Yeah you can settle that boiling hot nightmare planet, but it's going to be hard to make a profit. Maybe you find some tech later that now lowers their hazard rating by 50% if you can get a specific structure built (which again should require certain resources). These should not be super common and never generic (no -5 to all types sort of thing). I want to one playthrough have to search for nice planets, but maybe another I find something that makes barren worlds way easier to colonize and now i'm looking at them differently. Another I find something for gas giants and get extra lucky and a second one for lava. All of this leads to a less solved minigame, which right now is "look at hazard rating first". That's still a fine starting point, but as the game goes on I should have to reevaluate what i've seen.
4. More stations please. This has nothing to do with anything else but god are they a great addition. It just kills me there's only 3 (sorta 4) types in vanilla. If you get the chance it'd be great to see faction specific variants and more. They're an amazing addition
5. Since there's a post on it- planet size should matter. In a 4x it's normally because you can get more population, and each population makes 1 of each resrouce, then modified by the planet itself and the upgrades on it. I don't know if you want to bother with a full population system (it's kinda messy) but you might want to just do a simplified version, where a planet has say 1-5 production points depending on size, so larger planets get a larger modifier (eventually, colony needs to grow into it).
Alternatively you could have planet size specific industries, or just make it so a smaller planet can fit fewer industries vs a larger one. There should ideally still be a reason to care about small planets through, rather than just hoping for the lottery to pay out with a large terran.