Keep in mind that OP’s project is already based a different protocol than the Fediverse for their own reasons. Trying to create and maintain a bridge between different protocols might be more work than to just make modifications to posts in the current system.

Other factors are end user experience and branding. Keep in mind that the average Facebook/NextDoor user isn’t tech-savvy, and could also be put off by the weird software names commonly found in the Fediverse

It’s likely worthwhile for OP to look into flohmarkt, but integration might not be the optimal method