I haven't used Kagi much, but my understanding is that Kagi has their own indexing and you can customize your search by ranking your results.

SearxNG runs searches against many other search engines and then uses an algorithm to rank the results sanely. So less customizable but also the net you're casting is much wider.

You could easily self host on a free-tier instance in Oracle cloud or AWS for a year, or even just run it on a laptop. But if you really can't see a way to do that you can of course use one of the listed instances, you'll just be more likely to bump up against rate limits since you're sharing limits with many other people.