A lot of it comes down to which pens you happen to have - I’ve had some success with Sakura gelly rolls for white, and also more recently have been enjoying sharpie creative acrylic markers which has a moderately opaque white ink. I’ve also had some really frustrating experiences with some other pens and instruments!

JetPens has been an invaluable resource for learning about and buying pens. Their blog is great: https://www.jetpens.com/blog

Get some platinum preppys and some 'converters' (they both cost about $7, but the converter means that changing out inks is much less of an issue) and the whole world of fountain pen inks is your oyster... and there are some really interesting inks out there.

Also an interesting instrument that i've played with. Japanese refillable brush pens. You can get a really fine line out of them if you position them just right and great flow. One of the toolheads I built for my custom plotter uses a stepper instead of a hobby servo for pen lift (originally just so i didn't have to hear those little gears grind and for reliability) and i've been working on code so that I can vary the 'z' over the course of a line for use with brushpens.

White is tough, and I've never found a white ink that doesn't just fuck up a fountain pen... sakura gelly rolls are really the best that i've used, and even they can be testy.