While you're waiting, if you need to connect to a 1.21.5 server, use a backport version of ViaFabricPlus. I've been doing it and it has worked perfectly fine for me
Hi there, yes there is a way to download aristois as a mod, use the Installer and specify the version then click the launcher part turn it from vanilla to multimc/prism
You can update the Fabric loader version in the json file in the version folder.
I don't think the java server thing is related to Aristois. I am able to run the mod with 21. You can try changing it in the installation settings though.