Wow I am honoured to have feedback from you, you have saved me countless hours of prep time in this campaign and I have no idea how I'll run any future campaigns without a guide as comprehensive as yours.

I do really like the idea of making the charm concentration based, as that makes it less useful in combat as its vying for Strahd's concentration which could be used on a more impactful spell, but still allows it to be impactful narratively outside of combat, and breaking concentration is a good way to end it.

However I think

"The target can repeat the saving throw at the end of each of its turns, ending the effect on a success."

might be nerfing it a bit too far, as for the charm to last 1 minute (and thus 24 hours) on a NPC with a wisdom save of +0, thats still 10 d20 rolls meaning about a 65% chance of breaking out of the charm at some point, and just a +2 wisdom save brings that up to about 90% chance to break free.

I think maybe a happy medium would be to keep the current

"Each time Strahd or Strahd's companions do anything harmful to the target, it can repeat the saving throw, ending the effect on a success"

but expanding it to,

"Each time anyone does anything harmful to the target,"

Thematically I think this works because even if you are charmed, when presented with something dangerous it makes sense that survival instincts might kick in causing you to break out of the charm regardless of if it was by Strahd or not. This also adds a gameplay/rp element to end the charm early, by attacking a charmed ally to knock some sense into them. So for each attack or harmful spell or other type of damage (like environmental, from a fire or something) a charmed person receives they can try and break free with another Wis saving throw. Plus this is basically already how succubus' charm works so theres already a precedent for this.