Having multiple identical posts in different groups with distributed replies doesn’t feel ideal to me.
I think this is actually a feature. You’re essentially trying to centralize communities, but communities are decentralised just like instances.
Why do we have multiple Technology communities? Because some people might like the mods or the rules in the other community better or maybe you can’t even access one of the communities because your instance is defederated from the instance with that community.
Just as one admin doesn’t have monopoly on the Fediverse, no mod has monopoly on a community.