I've been giving this a lot of thought since my last post as the OP's feelings resonate with mine.
We are in slightly different situations as the OP's insurance brings in an income similar to if she had a FT job and has had SAH status forced upon her whereas I chose to be a SAHM and, consequently have no income.
I always planned to go back to work once the DC were older, however, the timescales got heavily altered due to illness and I know find that I'm unable to return to my career. I've tried minimum wage type jobs, such as retail but find that being treated like shit in terms of being expected to be available 24/7 but on low hour contracts with shifts being cancelled or changed at the last minute too frustrating.
I think that, for me, and I suspect the OP, being valued is the issue. By volunteering, not only is the grey matter active but doing something worthwhile makes you valued and feeling that you are contributing to society. It is also contact with other people.
The OP's DH describes her as neglecting her "duties" which I find deeming and she doesn't feel valued, she feels taken for granted and maybe even a 2nd class citizen within the household.. It doesn't take into account that the OP is also contributing to the household income. Technically, I would argue that, because the OP has become disabled but still has an income she could be entitled sit on her backside between 9am-5pm doing nothing while her money still rolls in. She would not be doing her "duties" if she was out at work. Her DH is not the only breadwinner. Obviously, this would be crazy and,of course the OP is going to take on more "duties" during the day meaning that the DH has his share of the duties reduced in the evening. Rather than seeing this as a bonus, the DH is seeing this as an entitlement and even thinks she should be doing more to give him even less of these duties.
The DH is no longer seeing the OP as whole person who, despite having disabilities has other needs that need to be fullfilled - maybe even more so now her life has changed irrevocably. His attiude says "I don't give a shit about your MH, feelings or ambitions". Her volunteering is NOT a hobby, it's vital to her wellbeing and her DH should be facilitating and encouraging it.
Stand your ground OP and tell your DH not to be such a condescending arse.