I’m thinking of making a redirect of the main game (join it, and it redirects to the platform-specific version, in a place). Does it break the TOS and/or XBOX guidelines? I’m not exactly sure.
tl;dr
A redirect to a platform-specific version so you play with other xbox, mobile, computer.
This isn’t against the rules at all. If I remember correctly, the 2017 Egg Hunt went so far as to move all mobile players to a mobile-friendly version of their game.
Not sure why you’d think this would be against the rules.
As others have said, it’s perfectly fine to do this.
But I’m going to go a step further and say if your game depends a lot on skill, then you should DEFINITELY do this. Capability based on platform is very real, and putting different platforms together may give one platform an unfair advantage.