I've been thinking about this re you -
not that I have solutions, just talking.
I think you do well working with yourself and few or less others. You have 'get it done' modes. Adhering to a boss strikes me as making you impatient for probably-sometimes good reasons, even if you could find one.
The thing about instruction re staying fit - does that absolutely have to be hands on as something to teach? Could you "lecture" as yourself - you have videos, and we know you can palaver. Maybe you could talk about it, including re your own neck and what people can do, including those with iffy necks.
Not that that is an immediate winning idea, but there might be a niche. In association with licensed physical therapists?
An a2ker visiting me, very capable and well meaning, suggested I bake and sell cookies - my reasons for not doing that are a thicket of complications. Besides, if I could sell cookies, I could sell a lot of other capabilities - there is just a wall of impossibility, which like you, involves some physical stuff.
I think your asset is your mind (you brat). Online. A website. Radio. An audience.
The bottom line is that being employed in the music business is probably not a good long term single career choice.
For example (as you may remember) I've done a high-tech solo act for over 30 years singing and playing guitar etc, but also balanced it with being a journeyman electrician and investing in equities, fixed-income, and real-estate.
For the last four year I've been teaching the four year electrical apprentice program full-time at a sizable educational institution and have done exactly one weekend gig earning $1000.
I'm 53 and no longer have the inclination to perform regularly for income, especially with the demands of teaching.
When I was young, my grandfather (who was the second chair violinist for the Toronto Symphony Orchestra) advised me that you need a good trade, in addition to music, plus you need to invest your money wisely.
You need to do similarly despite your age and other concerns; all things end and change is inevitable.
Typo: I'm 56 years old not 53; I wish you every success, but you have some hard choices and work ahead of you.