I'd like to use SearXNG as well but experience the same - I've tried a lot of different instances and settings but I always seem to get worse results than searching directly in the source search engine, for some reason? (note I don't use Kagi so this isn't an endorsement for them either)

They weren't my instances. There are some public instances floating around so, before trying to self host, I gave them a shot. I can't remember the specifics well. For me to bother investing time testing, I may have had a query that was irritating me on Duck Duck Go and Google so it might not have been a particularly fair test