I would counter that since you only have 1 reaction you are only holding 1 baddie at bay. Therefore your greatest method of restraint is to be within 5 feet, providing disadvantage forcing the baddies to attack you. if they are attacking you they are not attacking your friends. if they are attacking you you can defend yourself better. Additionally since this proposed build is utilizing forge cleric rather than paladin your saves are your weakest point. the +4 to a failed save will be much more useful than a single sentinel hit. Ie Mage who is nowhere near you casts Heat-metal on your nifty plate armor, or hold person you fail the save. well you are now basically out of the entire fight. or you pop a reaction +4 to save and you can keep working on keeping the troll away from your friends.
Your statement - resisting a save-or-suck spell you would otherwise fail vs. a single round of keeping a foe close - is generally true. But the frequency of them coming up makes a big difference in utility.
I'll say that being very hard to hit, most foes will not want to concentrate on you. Getting up to one and marking them (soft forcing them to concentrate on you if they stay adjacent) is an every round occurrence. Especially when you closed with ranged combatants (like a caster).
Now, let's look at how often do you fail a save-or-suck spell by 4 or less? First, how many combats are against casters? 1 in 3? How many save-or-suck spells do they have? 1-2 per combat? Of the three big saves, CON you have proficiency in plus a high CON, you're not worried about those. DEX does damage, you've got plenty of HPs - they aren't save or suck. So it's just WIS spells. Chance it's a WIS spell or one of the minor saves? Let's reduce it from 1-2 per combat to 1 per combat. What's the chance that they decide to cast it and to target you with it? 50%? So you'll make, some you'll fail even with +4. The chance the +4 will help you is 20%. So, 1/3 of combats will have a caster with 1 appropriate-save-targeting save-or-suck spell, will use it AND target you 50% of the time and you will fail your save by 1-4 points 20% of the time.
That works out to be that +4 helps you 1 combat every 30 combats. Vs. the reaction for Sentinel being useful once a round, every round. If you're going to take wizard levels might as well, but that requires INT 13 which drops other ability scores. I don't see it as hugely useful.
But looking at it like this does make me less worried about the number of reactions it will take up.
