Expert, depends on the person you ask, everybody are seemingly experts today. I would say 2150-2300 Fide+ at least to be called a chess expert. 1800-2000 are not experts imo, more like (strong )hobby chessplayers(unless they are talented kids). They can only see the tip of the iceberg and lack the deep understandig of higher rated players. To most GMs a 1500-1600 Fide would be not much more than a beginner. Most probably went from 1500 to 1800+ in less than a year when they were young, and don't remember how hard it was to learn chess when they were 7 . to have 2200+ chess probably has to be not just a hobby , but a passion.
1800-2000 FIDE are just hobby players?!
But don’t you think a player of 2000 FIDE strength is a expert? Like 2200s are masters they generally even get titles!
me with my 2093 fide rating:
Is your fide provisional?
If you start at a young age, and you study all the basics (let's pretend you study chess like it's a subject at school, which means regular and systematic study), then you can easily get a chess title before age 18.
That's why 2000 or 2200 could be considered merely knowing some basics, like @daat said the difference between a highschooler being pretty good at some basic math, but is nothing like a professor.
But of course almost no one studies chess like that. Sure enthusiasts (like me) will buy and read a chess book from time to time, but I'm not trying to build comprehensive knowledge. I'm not playing 3 to 5 hour games every weekend. It's just a hobby.