I'm a rank 402, currently at 8100 minerals / hour, and upkeep of 2.1 B with builder.
Can I safely remove builder profession for something else? Or how much would I need?

You can conservatively estimate 31K credits per mineral point, which means you should be netting over 6B credits a day. Depending on your colonization/invasion/construction/module needs, you should be able to switch without any hiccups.

as a 500 my mineral returns are 42k per point

Out of curiousity, how long have you tracked that? The averages I've seen passed around have always been in the thirties, even from 1K+ players.

Don't forget that there's the ability that halves the tariff, that affects your ratio.

well at 444 i have about 15k mp/h , and i make about 6B a day after my 7B upkeep. You should ideally be making double your upkeep in mining, so that you still are making a nice profit.