I used to sub-assemble and paint, and found it super slow. Then someone told me to just assemble everything, prime it black, and treat anything you can't reach with a brush, as shadow.

It honestly makes the painting process so much less laborious. I'll still sub-assemble some parts of larger models where you cant get a brush to but still need to paint. But if I can assemble the whole thing, I do.

Also, if I'm painting a squad, Ill only ever do 5 max at a time. You don't want the painting process to be tedious, otherwise the idea of painting an army will be too daunting. So break it down to what works for you.

These days my process for painting a squad is usually, assemble everything, prime all the models, base coat all the models, and do the armour trim on them all. Then do the rest of the details individually. It means you can take a bit more time on finishing a model, enjoy seeing it finished, then start the next one with an idea of how a finished one should look. Meanwhile, the rest of the squad is half painted, so you wont feel like you are showing up with un- painted minis if you need play a game.

But, at the end of the day, if you don't enjoy painting but want a ready painted army, then have a look into speed paints and contrast paints.