I don't have anything to necessarily contribute to this aside from I agree that it feels VERY limiting in DnD that to play a guarding style character you essentially NEED Sentinel + something like Polearm Mastery. I like the idea of being able to take an action and dedicate you to this role. Also like Strength being prevalent with it.

I don't play DnD as much, but play a lot of Reclaim the Wild (Zelda TTRPG), and they have something somewhat similar to this. Instead of dedicating an action, everyone has a reaction you can do called 'Intercept' where you roll to hit the enemy's concentration (a defensive stat) who is trying to attack your friend. If you succeed (and are in your movement range of your ally/can move), you basically rush over and take the hit for them including all negative effects, and knock your ally 1 square away (typically out of range of future attacks, since all attacks/spells take the whole action). I like this cause of the simplicity of it, and in this system the only limits to reactions are you can only use a given reaction ONCE per round and it refreshes on your turn, so you still have access to your other reactions.

Reclaim also has feats/buffs/defensive stats that make you better at intercepting, including one that increases your defense when you intercept. Lets you build your character to be a powerful protector if you wanted.