Asisreo
Patron Badass
I like designing tier 4 adventures and I think this is one of the biggest mistake a DM makes when creating their world: they don't add enough strong enemies in their universe (not just material plane).I wonder from your responses just how familiar you are with tier 4 play and what those spells can accomplish? My PC would not be very concerned, TBH, because there aren't that many NPCs or creatures out there she is likely to encounter that would defeat her. Worst case scenario, she has to flee to fight another day.
(This reply is an aside btw)
It's easy to always think low fantasy because the more grounded the adventure and encounters, the easier it is to imagine the scope for many. Where high fantasy takes a good bit of thought. If there's so many dragons in the world, why don't they eliminate human civilizations before a knight comes to slay them? If there's so many 10th+ level casters, why don't they just form a coup and conquer the world?
And the answer depends on you, as the DM. But the simplest answer can be the easiest: they are doing exactly what you'd expect, they're just in the middle of it. If a group of spellcaster should be able to conquer your world, make it so that they are planning to. Why haven't they done it yet? Because they need to be organized, make sure there isn't real opposition, be careful around other factions that might have a secret high-level force behind them. This stuff takes time, so while the adventurers are killing goblins to giants, the underground spellcaster organization has been brewing and they're starting to put their biggest plots into action!
There's also the different planes where powerful creatures of high intelligence have already captured their regions and isn't interested in the material plane (yet...). It's quite fun to come up with not just a city or country, but a multiverse where the different planes intersect and the players become a major component of the gods in order to keep the peace between the planes (gives an excuse to give players like martials and whatnot the ability to planeshift with blessings, charms, and boons of planar travel.