By 'swiftcast proc', I'm suggesting that there would be certain specific conditions under which Clemency could be situationally be weaved as an ability (ideally for free), such as immediately following a Holy Sheltron mitigation, for example. This would still allow you to hardcast it in emergencies when you didn't have set proc available, while also having access to a set number of free 'ability' casts per minute. If you want to expand on this sort of system, you just need a couple of utility/defensive effects that share a common proc, and let the player choose what it gets applied to.

You never want to design an action that always punishes you when using it, which was the problem with many tank actions pre-Shadowbringers. It ends up being dead hotbar space when you're playing optimally.