On a superficial level that seems like a roughly correct ranking in my experience. On the other hand, I picked up one of the category 3 languages pretty easily. I think some of these are more "weird" to a native English speaker than "hard" per se.

The aspects that make languages difficult for a native English speaker vary quite a bit with the language. I would expect individual experiences with the languages to have high variance as a consequence.

It seems like an extremely coarse classification. Category 3 contains languages with very different degrees of difficulty, while Bulgarian and Russian are both Slavic they are nothing alike in terms of difficulty since Bulgarian is the most analytic of Slavic languages (has the less inflection). That makes it extremely easy to learn compared to Russian.

Difficulty scale looks about right.

As others hsve pointed out, it's a very coarse (and rather arbitrary) categorization.

E.g. both Turkish and Russian are in Category 3, but Turkish is trivial compared to Russian.

Turkish grammar is extremely regular, and follows easily defined rules that fit about two pages of easily digestible tables.

In comparison, Russian is a separate class tought in Russian schools for four years to native Russian speakers. And you still get people who can't properly inflect numerals, for example.