My problem is not enough hot water mix, even though the anti scaled adjustment is set to full hot. This diverter is within a tub spout shower mixer valve.
What is the difference between Symmons TA-25A and TA-25B diverter spindle?

The TA-25A is the white spindle... it is used for the symmons combination tub and shower valve. The TA-25B is the black spindle... it is used for the symmons shower valve only.

If you are having issues with hot water supply (volume), then remove the TA-10 balancing spool and soak it in CLR for a half-hour... rinse and reinstall. While the spool is out, pulse the hot and cold water on for a second to clear the lines of any sediment that may have built up in the lines.

If that fails to fix the issue, purchase a new TA-10 balancing spool and replace that part... should resolve the issue quickly/easily for you!

Mark