Could it be that there are not enough Apple Farms? The further I am from the center, the fewer I can find.

At the beginning there are some additional farms. After that the spawning should follow the same randomness, no matter how far your are from the center. Feel free to share a screenshot of your map.

https://prnt.sc/Hf7FT8BCysyH

That looks roughly like what I'd expect. Gotta admit that it's not balanced yet, esp. not in comparison to where and how cities spawn.

I'm playing with the space exploration mod as well, and when I scan the planet I can see that the spawning of farms is somewhat steady. But in vanilla you have to explore the map differently, that makes it much harder to find resources.

Maybe funding exploration with currency is an idea to help with discovering farms? You get a new building that you can provide currency to, and it will then discover new chunks of the map, and some of them may have farms.

from a certain point it should be possible to build farms yourself