I made a similar creature that always imposed Disadvantage on all attacks that it can see. More specifically, it had permanent Foresight on it. I upped the effective AC by 4 (for the purpose of determining DCR). It was a little bit on the overpowered side, so perhaps a +5 to effective AC would have been more appropriate (I did not playtest the creature again after seeing it in action once).
For your creature, since it doesn't have Advantage on Saves, just imposes disadvantage on attacks, I would give it +2 to effective AC, modeling after Superior Invisibility as a trait (DMG 280). Raising its effective HP will make it too weak as it is still vulnerable to effects that require saves. You should note, however, that Superior Invsibility is more than just imposing Disadvantage, a creature with that trait cannot be targeted by most single-target spells that require sight. Having said that, I still think it's worth a +2 to effective AC.
As always, playtest, playtest, playtest! Number crunching isn't the final step to determine CR, try to pit it against some generic PCs and see if you need to adjust.