That is a nice looking ON-Q job there. Hats off to that. Could you get us a picture of the top of the panels? Can't tell but would like to see how many romex cables per connector you have there. Nice neat work otherwise.
I was thinking the same about useing a back to back on the 1st picture. I try not to use 2" LB's for 3 3/0. I think it would have been a pain to pull thru those fittings. I will use a mogal type LB because they will fit the conductors without fighting, scraping, and pushing with objects.
Myself I would have used a 400 amp pedistal (the utility calls it a meter raceway) with two 200 amp breakers in it. One piece and your done. It would cost more at about $1200 for the part. The utility wants the pedistals for resi because of ground movement from the seasons can pull the conductors and cause a slow self destruction. The pedistals allow the conductors to be zig-zaged giving extra length for movement.
Did you build that inside wall for the electric?
The fitting under the panel I would have used a 90 or a j-box to make pulling easier.