The eye can only focus on two of the three objects you mention..The rear sight, the front sight, and the target is a blur, that is the correct sight picture...This is basic....

I also, like you, wear bifocals...I use them to shoot and I have no problem with iron sights, actually prefer them on DGRs...A blurred target is to be expected so get out there and practice, it will all come together...

Don't let the scope crutch beat you up..