As a tank with a shield I want to be adjacent to the enemy units so that they don't attack my crunchy guys.
This also lets me gang up when my next unit moves and attacks.
Shield bash pushes them away which is the last thing a shield user wants. Give that ability to club users and give shield users an accuracy or damage debuff.
Probably it should be a damage debuff because it is already too easy to get 100+ dodge.
consider making shield proficiency something that assassins don't get because they are the best dodgers and shields help them get to insane dodge levels.
Consider this dodge math:
If my opponent has 100 accuracy and I have 0 dodge then a shield with +5 dodge makes me take 5% less damage.
If my opponent has 100 accuracy and I have 80 dodge then a shield with +5 dodge makes me take 25% less damage.
Because of this the dodge class is the best shield using class by far.