if you work with your proshop to develop layouts that accenuate the two new ones with what you want them to do then that arsenal actually sounds really good (in theory of course lol)

For top end, I would go 811c/t or the 706t.

As for middle ground, look at the 508a as a step up from the 300c, but less than the 505t.

Another option is to go with the 716c instead of the 508a; I have both and the 508a is about 4-5 overall weaker than the 716c, and starts about 4ft later (both are drilled nearly identical, as I wanted something similar in reaction/drill but using the different balls to change reaction).

How would the 508A compare to the 503A?

Im going through an arsenal change/upgrade.
Currently working with a top driller/gold coach.

What I have learned in the past month is that you do want some overlap in your pieces.
So that you aren't forced to change balls in the 8th frame of a game, you can stay with it until a fill ball or 1st frame of next game.

I have been through many drillers & coaches in my years of bowling and it really pays to find the right person for proper fit (my grip was not correct-felt Ok, but not allowing me to come out of it smooth)
Just so happens I have found 1 person that can do both.
My game had plateaued, I needed answers or I was going to accept where I was at.
Now, with a new grip and  a few coaching sessions, I am looking forward to seeing where this will take my game.
Maybe better, maybe not, but at least I know that it is me and not anything else