Thanks for the replies, everyone.
If Stubby disables EDNS by default and Steam probably requires it (due to their aforementioned regional pricing and geo-blocking practices), that's most likely what's causing issues with the service.
I won't be using Stubby anymore, since
@RMerlin's latest firmware already has a DoT implementation. For that, I'd only have to select a resolver that supports both DoT and EDNS and that's it?
Yeah, that's the same Wikipedia article I listed before. Thanks for the further research,
@Butterfly Bones. Guess I'll be limited to Google's DNS, then, as it seems to be the only one that supports both DoT and EDNS and also has datacenters in my country.