How lucky you had the shower curtain on this time, but it's still frustrating it happens again. Since you ruled out bladder stones etc., it is most likely behavioral, stress-related.

The Royal Canin Calm that Heidi mentioned, Fister loved that, but the vet said he shouldn't eat that only - can't remember the reason.

Could it be that Galaxy doesn't like the litter? When I looked after Oscar, his American owner always brought a special Eco litter, it looked like fine oats, beige colored. I found this that looks like it. Perhaps try that.

https://www.nuenergy.org/4-green-cat-litter-tips-eco-friendly-pet-owners/

Good luck and hugs to you - kisses to Sparkler and Galaxy.