Several maps generated with different seeds and all default settings results in the oil ocean covered starting area with some patches of ice and a few areas from Vulcanus. Is there any way to tune it so the starting area would be a mix of Nauvis, Gleba and maybe Vulcanus/Fulgora, that is, the earlier planets being used to generate the starting area instead of the last one?

You can turn off Aquilo in the settings.

But that's not even a half measure. I do want it to be present on the planet, just not let it take over the whole starting area. I'm fine if it takes up a certain portion of it.

I don't plan implementation for that, I'm sorry