I was checking the guide on how to speedrun Sonic Adventure, and it said that BetterSADX was not allowed. So what about SADX Mod Installer? According to their website, it says it converts the Steam version of the game (the one I have) into the CD 2004 port of the game, which has mods allowed for it. So would this be a good alternative to BetterSADX?

If you join the discord and look at the pins in the #resources channel, you will be able to find the SADX Coupon, which has everything you need to start running including the mod loader with a bunch of pre-loaded mods that are allowed for running as well as a few misc practice mods

please stick with the original disc version with the tools provided or the vanilla steam version. As Slysonic said in our Discord we can help you get set up, or also feel free to contact me directly through DMs