Agreed; it's completely impractical for someone to "hold" a proper non-modern war bow at full draw. Every single instance I've seen of reenactment archery using an authentic bow has the archer smoothly nock, draw and release in one motion. There is no "aiming" like we see in modern competition archery - this occurs during the draw. Not to say these weren't accurate, some video I've watched of reenactment archery looks positively lethal out to about 30 metres or more.
You wouldn't hold the bow drawn. The archers would nock their arrows and at the command would draw and loose the arrow. That would result in them all shooting at roughly the same time. Just a second or so delayed from the command.
I would think it would work to drill on draw speed (where the faster archers paced the slower ones). So you'd be able to tighten up the release window quite a bit vs doing nothing.
Bows have highly variable “lock time”. There is no trigger and you can’t reasonably get an army to draw at the same rate.
Reload times are fast enough that the fastest archer may be able to be ready to fire again before the last archer has completed their first shot.
In those circumstances, you would simply get orders to start and stop firing.