There’s probably a point where “diminishing returns” kicks in and they can’t get any higher by themselves after enough time. They also seem to keep whatever magic/skills they were specialized in prior to turning.

All of the Mer races besides Orcs have powerful enough magic and can live absurdly long lives too. The Dwemer could extract power out of a dead god’s heart, even, before they vanished. A surviving Dwemer (they are NOT dwarves, this is emphasized by several ingame books) could probably work and there’s always an ancient high elf if you need a cop out.

Or if you want a classic, nothing is stopping werewolves from becoming sorcerers in Elder Scrolls. You don’t HAVE to restrict yourself to being a basic nord to become one since they originate from a Daedric Prince. A magician could live just as long as a vampire and be a werewolf too as long as they stayed in Hircine’s favor.

They aren’t limited to being wolves either. Werebats are a thing and commonly used as guards by Vampires in Valenwood. Just about any normal or mythical creature you want can be a type.