I had known about Myers-Briggs for years and only recently discovered Enneagram, too. Any of the books by Riso/Hudson are good (Personality Types is a good place to start). There are also podcasts. Enneagram For Idiots is interesting if you can handle the hosts' banter.

This guy's Type 4 profile is probably my favorite that I've found online.

I am not an expert, but I find the online tests to be... inconsistent. And now, as you said, you have a confirmation bias.

I did a quick search online for the bullet lists that describe each type but didn't find it. If you look you can probably find it. I used https://www.amazon.com/gp/product/B004MYFLAY/.

I narrowed it down to the possible types, went thru the bullet lists and checked off what applied to me. It was clearly down to 5 or 8. At that point I read thru the detailed description in the book of each type and looked at the internal motivations descriptions.

Most folks who know me guess off the top that I'm an 8, but in looking at the internal motivations descriptions it became clear I was a 5 that was just well integrated.

Anyway, that's how I'd approach it. An expert may come along and give you a better approach.