By modifying the config file "reimus_initial_dimension-common.toml", you can easily change the initial dimension where players will be spawned when first entering the game. The default config value is "minecraft:overworld".
This mod is a fork version based on another mod of MIT License-Initial Dimension
And it has fixed issues below:
And for vanilla issues, /setworldspawn command can now be used in any dimensions(in vanilla, this was fixed until 1.21.9, but this mod backports this to 1.20.1)
If there are any more issues, please report them to github