If you want to be able to add custom engines directly from the settings, in about:config
add browser.urlbar.update2.engineAliasRefresh
and set the value to true
, after that you should see Add button in about:preferences#search
Edit: nvm, there doesn't seem to be field for suggestions...
Edit2: There's also https://mycroftproject.com/, where you can try to find the search engine you want.
Forum users tend to prefer long-lasting discussions, which is not the case on Lemmy/Reddit, where threads are forgotten after a few days.