Is there
any sunlight that gets in at all? Because in my experience, even the tiniest sliver of sunlight can photodegrade toys. In 2016 I moved my toys to a new TF room where 99% of incoming sunlight is blocked out. But just recently I looked at my Jetwing Optimus Prime toy, which was standing near where a tiny sliver of light comes in and doesn't even directly hit any toys... and his smokestacks were yellowed. If you look at my
Bayformers Optimus Prime vs Megatron TFTM re-enactment, you can see his yellowed smokestacks. Since then not only have I painted the smokestacks silver, but I've also added an extra layer of brown paper over that small gap where that sliver of light gets in. Note that this sliver was still obstructed by a closed blind and drape, but there was light bleeding from the very top near the ceiling that would bounce off the ceiling and onto the toy. I've blu-tacked the top of the brown paper onto the ceiling so that it now covers that offending top gap. In short, your toy room should be like a dark room... and then even darker.

Dark enough to develop film.
I have learnt the hard way that any amount of sunlight that creeps into a toy room, no matter how small, can discolour toys. Now I got MP Ratchet after I moved, and that toy has never yellowed at all. Only DOTM Optimus Prime had some yellowing, but only because that toy sits on a ceiling high top shelf near where the light was leaking in from. And even then, only the rubber smoke stacks yellowed; no other part, and no other toys. Not even my CHUG Sixshot and Horrorcons who have white and light-grey coloured parts and are also near that ceiling corner. I'm going to add another layer of paper over that existing layer just to make it even darker.
If you can, consider UV tinting your room windows, but yeah, definitely block out all incoming sunlight from any windows. Aside from DOTM OP's smokestacks, I have experienced
zero photodegradation since 2016.