Agreed this is a very tough decision for all the reasons you state. Remember that there are anti-discrimination laws, so our employers *must* be supportive, at least to the point of making "reasonable adjustments" to our work. Are you in largish firms (I'm not; a one-person charity with no money) are you in trade unions? How much if at all do you discuss what you can and can't - and could - do? M&S: the money thing is vital - have you talked to a CAB about your position now and in the future?
But I agree with your underlying thoughts: when to go to balance a) your need to work - for validation, for cash, b) your need to go when you want to go - jumped not pushed, c) your need to have a quality of life post-work (I shall be so fed up if my PD kicks in hard just after I leave work.
I'm still working on the equation . . .