One thing I realized is I don't really have a good idea for capacity that I need; I think I need to first nail down the different content load-outs I want and then find a pack that best fits all that kit.
This is an absolutely critical first step and is closely related to "How far do I need to be able to go to get to safety and under what weather conditions?"
After you figure out what to carry you need to rent or borrow a pack and actually carry it a short distance to get a feel for the weight - you will need to adjust your loadout (tendency is to overload), and you will get ideas about how to pack your load, not to mention what kind of ruck to buy.
Next you need to commit to a conditioning regime to carrying the load. This is way more important than the actual bag you buy.
An unconditioned man who buys that awesome (and expensive) 6500+ cu in Mystery Ranch ruck and loads it with 120lbs of unnecessary gear is going to get a nasty surprise when it comes to moving that rig... So will the guy who has not planned for living out doors and crossing terrain during a winter in most of North America.
My guess is you want a 3500-4000 cu in pack or larger for a BOB for 2-5 days give or take - winter or longer treks will require a lot more thought (larger bag? bike with panniers? skis/snowshoes with pulk? Kayak? canoe?).
One other thought: if your plan includes coping with a disaster where you might actually have to be evacuated on a bus or aircraft, you probably are not going to be able to carry much more than a 4000 cu inch pack and that might be pushing it.
To avoid attention, can you break down your long gun or get a folding stock?