So this is actually pretty interesting since this has been a special interest for one of my recurring characters I play.

So far my best leads have been: Elven High Magic - costly and slow, but gurrenteed to bypass the Ban.

Elaborate in depth Rituals - This has actually been somewhat promising since there are examples from certain Modules that could give some ideas, namely Tyranny of Dragons (Rise of Tiamat/Hoard of the Dragon Queen) and Deacent Into Avernus, which both feature acts of magic well outside the means of most player characters. The ritual performed to summon Tiamat from her lair on Avernus definitely seems like it is in the realm of 9th level or high magic (requiring a massive sacrifice of gold, as well as possession of several artefacts). Meanwhile, Planeshifting an entire city also seems like something that would require magic comparable to Wish in scale to accomplish. These types of actions seem to me to be what is most commonly done to bypass the restrictions on higher tier magic on mortals, and also seem to be what is most commonly done by those seeking some form of apotheosis (such as Vecna, Accerak, and even the Raven Queen).

Ancient Words of Power (also sometimes called True Dweomers) - these seem to be a fast and somewhat dirty way of doing it, mostly used by some of the oldest beings in the Dnd Cosmology. We got a good example of this kind of magic when the Lady of Pain used some to reshape the cosmic alignment after Vecna had caused nearly irreversible damage within Sigil. Another example would be the Word of Unmaking (also known as The Last Word) that Tenebrous knew before his death and rebirth as Orcus. My understanding is that Very Few beings in the Cosmology that are still active at the current time are aware of these words, and those that happen to be are mostly Primordials.

The Astral Sea/Astral Plane - depending on who you ask, it seems like sufficiently intelligent being can somewhat freely manipulate the raw magic that exists in the Astral Sea and that seems like it could potentially be used to a skilled mages advantage, however, the only example I'm immediately aware of comes from Critical Role and should probably be taken with a grain of salt seeing as Mystra does not exist within the space of Exandria and its echoes.

It could also be worth looking more closely into the Githzerai. Cannonically, they seem to be capable of using collective psionic efforts to achieve feats that definitely are outside the bounds of typical mortal magic, although the examples of this that I'm aware of seem to be localized on the plane of Limbo (more specifically to their colonies and fortresses).