TedTim - When I began regularly diving solo I made sure I always had my pony bottle with me. I learned early on that it was important to practice using and managing the pony bottle.
Coming in late here, apologies...
This is important. It is not enough to have the gear, you need to practice with anything new, whether it be gear or gear placement. If your pony bottle displaces some other gear, you need to get used to the idea it is somewhere else, while also learning the pony.
As for pony bottles, I think slinging is the best option for a solo diver, as if there is a problem with that gas supply you can see and fix it quickly. If it is on your back you may be SOL.
Learn to think about your gear more than you may already be doing. Alot of gear has multiple uses. For example, when diving on a wreck offshore, or in an area I might encounter boat traffic, I like to bring at least two surface markers. One of those might be a flag I leave at the entrance/exit, but the other(s) I take with me. One of them is a standard safety sausage, the other a lift bag. The LB does 3 jobs. First, it lifts stuff like bags of mussels and other stuff. Second, it is a redundant source of bouyancy if my wing should fail. Third it is redundant SMB should I lose my primary. So redundant gear does not mean you need to take 30 pounds of crap down with you, here one item counts as two sources of redundant gear.
My kit is based on conditions, not whether I solo or buddy dive. Certain sites I always bring certain gear. This IMO reduces confusion since you are not having to reach for something you had last dive, only to remember it isn't there now. I don't carry the pony on a 15 foot inlet dive, but I do on all deeper dives.
For me, solo diving is 30% gear, 30% skills, and 100% mental. (yeah, I know, I stink at math
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Bottom line is, can you rely on yourself to A) avoid trouble, B)recognize and solve problems when they do happen. Would you trust yourself in a major butt puckering situation? What I mean is, knowing yourself as you do, would you buddy up with a clone of yourself if you were a new diver?
Gear helps out some. Skills practiced in a pool, sheltered area, on a safety stop help some. But when it really counts, what is in your head is all you can really rely on. If you have never been in an emergency situation, well, make your best guess. Then plan and hope you never find out...