TwoSix
Everyone's literal second-favorite poster
I have no problem buffing fighters, paladins, and rangers at low-level. Outside of PAM VHuman shenanigans, they're not crazy strong.Yeah, coming up with non-attack bonus actions was the best solution I found as I've been typing stuff up. For example, for GWF, the bonus action allows the attacker to shove or knock the target prone. Although an attack, it does no damage and follows the flavor of pushing/shoving someone with a two-handed weapon.
But do you see how even your suggestion makes it OP a bit? Allowing Defense style to have the character gain the benefits of Dodge while still getting in one attack (all they would otherwise have up to level 4) for a bonus action they would not likely use anyway... Now all your enemies have disadvantage to attacking you but you still get an attack in--it is too much (for me, anyway...). Maybe if it allowed you to make an attack as a bonus action but with disadvantage? You are focusing on your defense after all.
To be perfectly clear, my balance concerns are almost entirely concerned with intra-option balance, not worrying about if the baseline efficacy of the party versus monsters is changed. That's trivially easy to handle via encounter changes.