They're completely incompatible systems. Tumblr was always more of a microblog while WordPress is basically a CMS, at this point. The only way to get those two to talk would be to federate, now that I think about it.
I’ve been second guessing WP deployments as well, but I think the most likely outcome is a community fork once Mullenweg’s brain rot affects commercial interests too much.
Ignorant question incoming: Would this result in a huge influx of traffic if your instance federates with tumblr, because you’re federating with all of tumblr? Or do individual tumblr blogs need to federate with each instance?