I don't have a definitive guide, but I'd suggest looking at these videos if you haven't already:

• Retr0bright - de-yellowing do's, dont's, and playing devil's advocate by Modern Classic
• Hydrogen Retrobright Experiment by Chuck Hutchins
• New Retrobrite Technique for Indoors | Sous Vide and other methods to reverse yellow plastic by RMC - The Cave

The last one seems like probably the best solution for countering the risk of marbling.

While I haven't watched it yet (I'll update this once I do), The truth about Retr0brite – busting myths with science… also showed up in my sidebar and tech YouTubers who I do follow have left favourable comments.

I don't have a single definitive guide, but I'd say these videos together serve that purpose well together.

• Retr0bright - de-yellowing do's, dont's, and playing devil's advocate by Modern Classic (The title says it all. For example, if you're doing it to try to make something more saleable, don't. Collectors can tell and don't want you taking that decision out of their hands.)
• The truth about Retr0brite – busting myths with science… by Hey Birt! and Hydrogen Retrobright Experiment by Chuck Hutchins (Scientific experimentation into how the retr0brite process works on a chemical level, with the first video following it with a section on potential gotchas and suggested ways to minimize them, including advice for how to do immersion+heat retr0briting very inexpensively.)
• New Retrobrite Technique for Indoors | Sous Vide and other methods to reverse yellow plastic by RMC - The Cave (An alternative approach using immersion and heat. See Hey Birt!'s video for what to be aware of when setting a temperature for this method.)

If you don't have time to watch them all, watching the Modern Classic and Hey Birt! videos should serve well as a minimum recommended set.