How would I replace the overworld dimention with my own

I think replace all the overworld biomes with my own and removing the vanilla ones would be to much of a hassle if its even possible but I am curious if it is possible to remove the overworld and have that default dimension be a custom one. For example, instead of a new world spawning you in the overworld, it spawns you in a custom dimension

Thank you, have a good day :)

also i am using 2025.2