Again, I'm not considering them to be intentionally malicious or deceptive, I'm saying without the basics in place, we're being asked to just trust them.
I'm aware of the limitations you describe and you're right that there's no way to 100% guarantee anything, there has to be some element of trust. So the services/software I choose to use have done all the things I mention, or I run them locally. Does that mean they're 100% perfect? No, of course not but the fact they've gone to great lengths to establish at least a basis for trust means a lot to me. Some of them have gone on to be tested in some sort of legal encounter where again, they performed well.
Trust is a personal thing, we all have different perceptions of what makes an org trustable - if Kagi match yours, good for you.