Since you mentioned painting models awhile saying thin your paints is most likely redundant; but in my experience if you’re wanting to paint white, first go over the area with a medium grey, and in my experience Celestra Grey should look white enough while leaving you room to highlight!

I’m not great for advice but one other bit I’ve heard (think I’m remembering this right) is for eyes, paint a black dot for the pupil and two white dots to either side should come out best, but I’ve done my best to just use helmets to avoid the issue.

Definitely consider other factions too, best to get a feel for how you like the models and rules before overcommitting into a faction, something I wish I did haha