This is already a very long thread and I'm not sure if anyone will read this far, but for what it's worth here are my comments. I apologise for repeating what has been said because I haven't read right through yet either! Up until the pandemic I flew a lot to and from the UK, especially whilst my mother was weakening and after her death to sort things out.
Backpacks
I find that a backpack if full of heavy stuff, such as on a hike or as a day at when on holiday, makes my neck ache and I end up with a headache. It is better for the back than an asymmetrical shoulder or crossbody bag, though. And, of course, as everyone says, it's hard to gauge if you're going to hit someone with it if you stand up or turn round, and you can't keep your eye on it - if something is taken you may not notice for a good while.
As you will need both hands to help your DH, though, it certainly is the best solution, so if you go for this, check that it has as many anti-theft features as possible.
Crossbody bags
I have never used one of these, although I often use a tiny handbag with a very long strap worn diagonally UNDER my jacket or cardigan.mthe bag always in front of me, the flap on the inside.
I am thinking of getting on of the ones described here by this lady, who has sensible advice (if you don't mind her accent!)
youtu.be/rCKHgLI40Dg
They are more expensive but with all the precautions taken in the manufacture of the bag, you should have your mind free to cope with the extra worry of caring for another person.
Conclusion
So I would go with the double solution - a crossbody bag under your jacket and a backpack for extra clothes but nothing too heavy. You can carry an empty water bottle through security if you are flying and you will be surprised what you don't really need while travelling. Keep it light and minimal. Don't clutter yourself with things that you can pick up as you go.
HTH and apologies once again for repeating what others have said. Off to read the rest of the thread now.