Hello Max
As a confirmed, but not proud, member of the turtle club (full accidental capsize inversion experience) I empathise with your dilemma about inflatable life jackets versus fixed buoyancy aids. The ideas shared here are offered only in the hope that more qualified people can say whether they are sound, or not, so please don't think I write with any great authority.
Unless very bulky, a fixed buoyancy aid is not adequate for prolonged support to a disabled person in the water, especially if fully kitted with heavy sailing gear. An inflatable life jacket on the other hand is so bulky once deployed that it is very difficult to swim or climb back onto the boat to right it. Consequently, as you suggest, I have been using a "belt and braces" combination of both - but although I have been wearing the combination for a few years I have happily not had occasion to test the system for real.
The attached illustration shows my inflatable life jacket, with built in harness, fitted on top of my sailing jacket, under which I wear a gilet buoyancy aid. The gilet I use is a 50N Tribord float jacket from Decathalon that I find comfortable, warm, good pockets and discrete enough worn black-side-out to wear to the pub without attracting attention from the fashion police. See
https://www.tribord.co.uk/izeber-50-floating-system-jacket-bp_24182 The inflatable life jacket is a CO2 inflated type but is manually activated NOT the automatic type that inflates automatically in contact with water. Apart from anything else automatically activated ones seem prone to unhelpfully inflating themselves in damp lockers, don't they T..?
The inflatable life jacket has a built-in harness on the principle that I can tether myself to the boat. In practice I don't do this because I'm worried about being trapped by it in a capsize or dragged along under water if I'm solo. If I am at sea with full water ballast I think a capsize is very unlikely and the consequences of falling out very serious in which case I have the option to use a tether. Probably I should also carry a knife in a more accessible place than my pocket so I can cut myself free.
The other point of the harness is that it has nice big D Ring which my trusty crew, passing boat, or whoever, can securely attach a line to before fishing me out.
To top it all I attach a FastFind GPS personal locator beacon to the harness webbing where it is out of the way but accessible and also visible to other crew if I'm not capable of deploying it. Hopefully it will never be used but from what I read, and from talking to people I respect, a man-over-board at sea is an SOS situation and effective signalling for proper assistance without delay significantly raises the chances of survival. Of course if I'm pootling about inland on Lake Bala I am unlikely to need it. I only have one, so I selfishly keep the PLB on my jacket but I'm ready to use it if any crew go MOB. If I'm the MOB I still want to be the one that takes the decision!
Does anyone want to quote about "duffers and drowning" from Swallows and Amazons? Of course - even though I kit myself up with more gear than Tom Cruise in "Mission Impossible" - I acknowledge the most important piece of safety kit should reside inside the skull of the skipper and safe behaviour and skill are the best guarantee of safety, not reliance on kit.
Matthew
BR20 Gladys