The Level Menu mod allows you to go to the mission or sandbox level selection menu during the game, which is currently available in the game.

To enable the mission menu, download the mod, activate it in the mods settings, go to any of the game modes, press Esc and open the "Level Menu" menu at the bottom
After the update, the author will add the ability to see a third list with all the installed maps, which can be launched immediately from any game mode and mission.