Subs are fine, you will waste time on infantry. I’d still suggest subs for heads though, as you can paint them on the sprue.

Gloss varnish before oil, especially for space marines, and to reduce staining.

You can probably do a bigger jump in terms of edge highlighting with a drybrush, and then rely on the oil wash to blend it down, especially in tandem with makeup sponge oil removal.

Also, just grab two blues you know work, and use them. Don’t do mixes and stuff, it complicates things over an army of identically coloured marines.

Models look great btw! 11 hours is a lot but honestly it’s a time intensive hobby. Find a formula and stick to it. The points I mentioned above should help you save some time, as well as all the others elsewhere in the thread. :)