From experience there are no perfect pens.
Option 1 - Oil based pens. Those are the ones with the mixing ball inside that you have to rattle. awesome coverage and durability!
I wanted to used orange one for my jetbikes, to draw in the detail pattern (
https://www.dakkadakka.com/dakkaforum/posts/list/0/760381.page)
But all oil based pens have big tip and the paint consistency pool very quickly, blobs, and dries superfast. It is very tricky and hard to control, ultimately ended up painting patterns with the paintbrush.
Option 2 - Gel pens. Those work better on some surfaces that others, but never any good coverage. So those wont work for mini.
Option 3 - These.
https://www.aliexpress.com/item/32995799085.html?spm=a2g0o.9042311.0.0.27424c4dyNczmK
For some reason these have good coverage smooth and detailed. However!!! I have not tested them on mini, on plastic, with other paint, or how the markers will react with matt varnish. Also, these markers are metallic, but there is a white marker in there too, it goes on almost milky clear, but when dries becomes white. For small details it works.
So far best markers I have tried.
Here is quick demo, goes on clear, dries white, but using marker over dried marks washes away previous marks. So this wont work for large coverage.
There are ways to improve shake in hand. So far, good paintbrush and well braced hands are the best tools for white.
Other options are whiteout pens but I have not tested those.