Just a consideration I didn't see mentioned, (or I skimmed over it, equally likely), is that some of this depends on genre. I imagine a military and some sci-fi novels are going to use klicks and they might expect their readers to know what it means.

Dialogue might leave out C or F when mentioning temperature because who says that unless you live somewhere like on the US Canadian border and have to clarify it sometimes. Otherwise you just say something like, "It's five below" or "It's ten degrees out there".

I too, having always lived in the US, found 'kilometers' awkward in my characters' dialogue. But I'm using it because it makes no sense to use 'miles' or 'Fahrenheit' in the future. Invented units would also make sense.

For straight narration I'm fine with writing out short numbers, numbers at the beginning of a sentence and using numerals for larger numbers. I believe it was what I was taught in school, which doesn't make it correct.

When it comes to C or F, as a reader I don't notice whether it's spelled it out or not, nor do I notice if one includes 'degrees' or not.