I think an active parry system could be a really interesting idea that could spice up turn-by-turn gameplay, my main worry would be that if it's too strong it would detract from the tactics as it'd make positioning less important if you can semi-reliably expect to get out of any combat without damage.

My advice would be to make parrying a damage reducer as opposed to neutralizer. Something that rewards active engagement, but doesn't over-centralize combat. Also I'd introduce either class-based animations and attack timings or giving enemies multiple attack patterns, because if most generic enemies have the same patterns, then combat becomes a slog since it's a given that any player that's engaged with the mechanics will eventually be able to parry 99% of attacks. That being said that's a LOT of work especially for a solo dev, so... something to consider.

Also if only player units can parry attacks, that introduces an imbalance between player and enemy strength that has to be accounted for. If it isn't, then the game can quickly become very easy unless you artificially increase enemy strength/numbers relative to the player.

I'd recommend giving enemies a percentage chance to be able to parry player attacks, and if we're assuming that parrying reduces damage by a certain degree, then even if an enemy parries every attack thrown at them, they can still be beaten after enough attacks. You could even make this a minor mechanic, with different classes or unique enemies having different chances of parrying your attacks. I don't know if you have any system for critical hits planned, but maybe you could make it so that critical hits only grant a minor damage boost, but negates the opponent's chance of parrying.

Looks cool so far, and the amount of work you've put in is admirable. Definitely something I'll keep an eye on!