For me, Usenet has replaced public trackers and competes with the private trackers I'm on. I find I'm mostly going to Usenet anymore and am only going to PTs if the release they have is higher quality or to keep my activity levels up, but most of the PT releases get uploaded to Usenet anyways. Occasionally there's some obscure thing I can only find on a PT, but it's rare. Getting a few good indexers will definitely increase your coverage.
You can preview the indexer only, just wouldn't be able to download anything without having a provider and a download client (like SABnzb or NZBGet). I think the provider usenet.farm has 10GB free trials or something like that if you did want to test it out.
I use Sonarr and the manual search available on the Indexer sites, which suits my needs. Haven't tested out any other searches.
But happy to answer any other questions you have. Feel free to DM me.
The dev did sanction the NZBGet fork tho (nzbgetcom/nzbget). So there is still a viable and actively developed NZBGet application for those who want it