I think that all lines are fine and the best line is given by specific villain's tendencies that we have no way of knowing. Bbb line is an option against very fishy players and I would not dare to go for the third street of value against perceived reggy player/ I think its just too optimistic.

I like bb xb exactly because we would always bluff this card. If villain is calling two streets, I think that he is somewhat more likely to call F/T. The counter argument would be that he might be afraid of another bet otr, if he calls ott, which is not concern if you choose bxb, but I'd guess that nl50 regs should be more sophisticated than that and realize that it is better to call a range that still has bluffs in it than to calldown a range with no bluffs. That said, against fishy/ showdown happy players the second argument might be stronger, making bxb line preferable.

Check back ott and bet otr screams SDV hand going for thinnish value, I dont think that you are ever bluffing here and I dont think you are ever bet/calling otr, which is a small concern, since almost nobody is capable of x/jamming as a bluff, but it is a concern nonetheless.
Those are my first thoughts.

Now I am thinking if we need/have xb ott/ call otr range. If we need it, KQ without a heart should be a prime candidate. But since villains flop x/c range consists of SDV hands that dont need to bluff (AJ etc) and few FDs that often paired up ott, I dont think we need to really worry about that range.

But thats just off the top of my hand, Im not sure.