When to move on from a drive? I do when about 10% left, haven't had problems but...

Technically what happens is that the operating system and running apps use empty system drive space as a temporary location to write temporary files. Like an extension of RAM (virtual RAM is the term). All operating systems do this. It is estimated that they need about 15% of the drive's total capacity as a minimum. Once you fall below that, very erratic and odd behaviors start cropping up. You also risk data loss and operating system corruption.

I never let my drives (internal or external) get more than 50% full though I believe 75% - 80% would be OK.

Drives are so cheap nowadays that I just get new ones . . . 2TB NVMes (Crucial, SanDisk, Integral etc.) in USB-C housings cost around £100 on Amazon.

You can pay more for the latest super-duper versions with much faster "claimed" speeds but you won't notice any difference in the real performance of your computer.

Assuming you mean external drives, you could probably push it down to around d 50GB without any ill effects. But at the end of the day, its your own decision.

If you mean the one you boot from, yeah you don't want to get it too low.

I agree with Phil0124 and would add that for the Mac's system drive, a minimum of 15% is recommended.