Hello there, so here's an update! The main addition is the Custom Game Menu, so you can replay levels/the full game with weapons/spells/upgrades/gameplay modifiers you've unlocked.
To unlock levels you have to finish them in the regular game mode, if you've completed the game already before this update just replay the last save in the last level, finish the game and you'll have all levels unlocked in the Custom Game Menu. If you don't have the next level finished when playing a custom game, you'll be kicked back to the menu upon completion of the current level. You can also enable a cheat menu, so if you pause the game you have access to a menu to give weapons/ammo/spells/health/mana etc.
As well as this a bunch of bug fixes and gameplay balances are in. Notably:

• Super shotgun upgrades carry between levels now
• Saves made at the start of a level are now separate and aren't saved over at save shrines
• Siphon mana spell gives the player more mana
• Weapon numbers can now be bound in the key rebinds menu
• Spells now have quick switch buttons (F1-F6 by default, can be rebound)
• Added a quick-switch button to change to the previously selected spell (Z by default)
• Crossbow now does higher damage to armored enemies
• Pyre spell particles have increased, making it easier to burn enemies