Butt-flare is quite common, yours is hardly visible but a bath might make it perfect, just remember to try with belt on, butt tends to "pop out" more when you close the belt.
The belt will most likely fix that.
Some suggestions besides what you already said yourself is: IF you want to go for it - I Think it looks great as it is right now, you`ve done great work
1# Shorten the strapping between butt/kidney
2# Move it closer to the ends
Last resort
3# Brackets and elastic create tension that will pull the armor right.