You could try expert mode but I found it annoying to play without mods. Enemies are really tanky, I think you deal 0.2x damage overall compared to adept or something. Fights weren’t that much more difficult, just longer. You will spam projectiles and run in circles.

Oblivion isn’t that hard of a game imo, I think you should just continue on your playthrough. If you start a new game you’ll still be very powerful around level 20.