I am experimenting with a new search engine, https://neera.ai/ and I want to make it my default search engine on Safari. Though Safari doesn't give me an option to specify in a new search engine. https://apple.stackexchange.com/a/69198/409600 is similar, but the accepted answer doesn't work anymore.
It's been suggested that since Apple disallows using other search engines (other than their own sanctioned options and/or the ones that develop Safari extensions), we could use something like this with Safari:
Recently I saw another option - and I CANNOT vouch for the quality of it, but it seems somewhat well received via reviews is xSearch for Safari. Again, I'm not a user and I cannot say if it's any good, but it seems to be a possible option. I'm guessing there may be other similar extensions out there that I've not yet seen...
Let's hope that Apple finally puts this power back into the hands of its users at some point soon instead of resulting to convoluted options!
So you could assign "n" to Neera.ai, "g" to Google, "duck" to DuckDuckGo, and so on, and per query direct to your search engine as indicated by the prefix.
I use it since 2 years and it works fine. Just occasionally after some Safari updates you may need to run a small cleanup procedure.