I'm not sure what percentage of user IDs you will see passing by. Another concern is however that the majority of the OpenDHT traffic appears to be originating at OVH-hosted nodes, not actual users. These should be able to associate your user ID and IP address.
Note that with a central server instance one has many of the same concerns - but compromising a server is a bigger hurdle than running a bunch of OpenDHT nodes, and there is also a higher chance that some irregularities will be noticed.
A Mastodon instance for info/cyber security-minded people.