The thing people are missing with base painting is that it should be a half-way house.
Ideally, all 4 players pick a different route out of the base. Let's say it's Port Mackerel. One goes high left, one goes high right, two up the middle splitting up once the map widens out.
The high route players ink the path as they go, taking a moment to fire into the open spaces at the places they can. The lower players go straight ahead, flicking their aim to the side to get at the gaps between the boxes.
Everyone comes back together around where the map opens out and mid starts.
Just like that, you've all pushed to mid quickly, built special, and inked the majority of your base.
It's a waste to do anything more than that, and you definitely shouldn't be religiously filling in all the tiny gaps, since about 10 of those is like... 1 shot of yours landing in the enemy ink. Fix any serious gaps of course.
Frankly, on 6/8 of the maps I think it's almost a moot point. Splatoon 2's spawns are so tiny I'm not sure it really matters. Only Moray Towers and Port Mackerel actually have much distance from spawn to mid anyway.