It's likely that whatever you buy, you're going to change it after a year or two. I bet most of us have done that. I've gone through several iterations of packs and racks, and will probably change again.
On my first tour in 1975, I'd never even heard the word "pannier" and I still managed to pack what I needed for a two-week trip around Lake Michigan, in a couple of duffel bags from gym class hung from an old steel book rack, the kind with the spring clamp on top. Eventually I geared up with the four panniers and handlebar bag for a couple of decades, and that just got too heavy. Now I'm with two small Arkel Drylite rear panniers and that's all. Notice the smile on the cyclist above with the light load?
Common wisdom is to gather your load first and buy the pack to fit it, but you probably don't even know what you want to carry, and it will change over the years. Thus the multiple iterations.