On certain days, like the weekend, youre getting this notification every X-seconds. And theres no way to turn it off or adjust the volume of this rather obnoxious sound thats played every time this appears. The notification itself is also rather big and obstructing..

I think there should be an option to disable this.

[ESC] >> [F10]"Settings" >> [0]"Settings"...
"General Settings"

> "Alerts"
--> "Black Shrine First Vanquisher Notification" (last item in the list)

As I understand it, disabling Black Shrine First Vanquisher doesn't work in this case, because this is not the first clear for a class and calamity level, and disabling that alert only works on the first clear, not regular calamity 8-10 clears. There might be another setting to disable the calamity 8-10 notifications though.

no, there isn't. That's why the op is suggesting.

Correct

I did not know that there is a way to turn it off. I looked through the game options but somehow missed the one that TheVoidSinger suggested - It was already turned off / unticked so it was supposed to be hidden.

But what you said applies to my case - certain (and in my opinion still rather annoying) notifications are still being played.
But some days its more, some days its not as much.

You can only turn off the "first time clear" notification. Most of them are "reclear" notification which you cannot turn it off. I don't know what was the intend here. Want ppl to show off they clear C8-C10? But those boss are easy meat even in day1. Want to announce there is a server buff released? But there is a special icon for it. I don't know. Typical PA behavior (you never know why).

Regarding your "some days more some days less". Since you only need to do it once per week, most ppl chose to do it right after or right before the reset (Saturday night). Similar to world boss.

And a toggle for the Edana blessing ones, too, please! I'm not going to be fighting anything in Edania but coral crabs for a long while yet, and they only do anything in their individual zones of Edania.