Your root mass just seems small to me for the size and age of your plants. You hear a lot of guys talk about that the health of the roots is directly proportional to the health of the plant.
This is a pic of the way I start my seedling out, before I transfer them to their permanent home.
View attachment 4606416
and a pic of the roots.
View attachment 4606417
This isn't the bubble tech that you are using, but pretty close. Keep the roots wet with lots of oxygenated nutrients.
This just has one scrub 360 degree sprayer head on a timed pump. 1 Min ON, 5 Min OFF.
(Not trying to sound like 'Oh look how big my roots are', just was showing you an example.)
Something seems off with your grow, but by the numbers, every thing seem spot on.
The only thing that I can think of would be your lighting, and that's just grasping at straws.
Have you tried taking a light reading? You can download an app for your phone. Won't be totally accurate, but will give you an idea. 15 cm up or down can make a big difference.
The Hempy buckets may be a good idea to get this first grow in the record books as a WIN.
I have never ran one, but I did read on them a bunch when deciding what type of system I wanted, they seem like a solid system.
Another system that you might like is call the 'waterfarm', a couple of my friends on RIU run that system, and have some amazing results.
They both have grow journal.
@Dr.Amber Trichome @Laughing Grass
Doc Amber just pulled 20 oz. (567 g) of one Blue Dream plant.
Wish I could be more help, Good Luck.
WillieP