There’s a surprising amount of classifying happening at the content level. If you have a clean ip but then send content that raises flags (includes hot words, adult content, etc) then you’re going to be categorized differently in the inbox regardless.

Even without the borderline content, if it seems promotional, it may end up in promotions regardless.

98% of email is keeping a clean IP range. Getting a clean IP is good for maybe a week if you don't know how to keep it that way.