Hello!
I'm MBC and I'm at my third i30 right now, I'm grateful to start my membership here, hopefully I can provide some useful info and I can learn from others that are, more likely, well much better informed than me.

I'm from Cluj-Napoca, Romania (EU). Here is a picture of my car a 2024, non facelift, PDE Hatchback with the 1.5 T-GDI mild hybrid and manual gearbox. In my country, the trim level is "Highway" which is the middle of the road right now as features. The color is Shimmering Silver.

Welcome

So in summary you have misting inside windscreen and loss of coolant. That sounds like a coolant leak, maybe at the heater matrix.
Normally any misting can be cleared using air-con, but not for you it seems.

Is there any sign of wetness on passenger side floor or glovebox area?
Any coolant on ground under car?

This may be worth a read: Heater Matrix blocked and leaking

When was the last time the Diesel fuel filter was serviced?

I'll be honest...I can't remember: it may have been last year, it may have been last decade! (Bad memory & the mechanic I use doesn't always give me paperwork).

I'll ask him, to make sure, and get it booked in if you think that might help. Thanks

Terry T.
 No need to apologise.  I thank you for your interest.