I love
Agnieszka! It's a bit of an '80s name but not terribly dated. I know lots of Agnieszkas, mostly born from the seventies to early nineties. Honestly if I was having a daughter right now I would seriously consider this name. The main drawback is that the full form already sounds a bit like a diminutive. Not enough to rule it out though and I actually find
Aga charming.
I've never heard of anyone going by
Agnia, it's not impossible but it is quite awkward to say, it just doesn't roll of the tongue - the g really doesn't want to stay there and I feel like it would morph into just
Ania in quick everyday speech.
I personally, and most people around me would break the syllables as a-gniesz-ka but it doesn't really matter as long as the stress is on the e.
I also find that the distinction between Polish sz and English sh is honestly a bit pedantic, I wouldn't worry about it. The more noticeable difference would probably be the ń vs ny but genuinely not enough for anyone to care that much.
https://www.behindthename.com/pnl/109883