I think the part that you need to think through carefully is how moderation policies may differ between communities or even instances. And there are a ton of reasons you might want to not subject users by default to comments from another (eg, a community aimed at a topic with “PG” comment moderation mixing with a similar community that is targeted more towards adults).
So, keeping in mind that the “social contract” of allowing community members to remain protected by the community rules that they signed up for should be something that can be preserved.
Not out of reach, though. Opt-in to per community xposts. Filter out by first non-allowed-community comment per comment thread. For sure do-able.
Just important to consider why we have communities in the first place and to consider the different types of stakeholders.
I voted your comment up btw
I’m on that honour system now