This is a simple mod that spawns the player upon joining a world to spawn in the Nether instead of the Overworld. Once you download this mod and play your existing world with it active it will teleport you to the Nether. The version for this mod is 1.19.2.
NOTE: I wasn't able to figure out how to spawn the player on land all the time and not in lava or in a block sometimes. If this does happen just start a new world or go in creative and put yourself in a better spot.
WARNING: Also, when you die in any dimension including the Nether, it will spawn you back in the Nether in the same spot from when you first joined the world. I could not figure out how to fix this problem, especially when you do reach the overworld and place a bed.
Even though this might not be the best mod out there, it is a very simple one. Now go out there and beat the game from the Nether!