Good to know that shoulder bags are still considered personal items. I've been using a Tenba DNA 'slim' messenger bag. My 15" (they come in sizes) fits a compact laptop, tablet, FF camera body with 24-84 lens attached plus two other lenses up to 300-ish mm. I don't fly regionals, haven't been on an inter-island for a few decades, but have had no problem bringing that plus a carry-on onboard.
Speaking of carry-ons, I've been using a separate shoulder-able bag *inside* the carry-on. I put toiletry kit, pills, one set of socks and underwear, plus anything with a lithium battery in that; makes it simple if I need to check that carry-on at the gate - and I've never had an issue carrying a messenger bag plus a smaller shoulder bag onto a very-full flight.
That said, I travel relatively 'heavy', so I'm almost always checking a bag. I spotted a Samsonite hard-sided no-zipper clamshell (they don't make these anymore) at an outlet store, grabbed it. When I fly with my wife, there's room enough for both of us in it. Packing cubes make it simple.