If your boundaries didn't work, you were probably doing them wrong

Boundaries aren't about making him understand or listen to you. They're not to make him respect you or stop nagging, or treat you differently.

One of the doctors I follow on YouTube defines boundaries as 'living into who you are. And defining for yourself what you will listen to/tolerate or not.' I like that definition of boundaries better than the idea of boundaries being something you enforce on another person. That doesn't work, so you just have to do your own thing.

I'm a bit like you in that I have limited resources and options, so have to bloom where I'm planted, at least for now.

If he nags that you're spending too much on food, and your gut tells you he's going to get on a rant, acknowledge it politely and unemotionally - "Okay, noted. I'll think about it." And then remove yourself to another area to do something else more constructive for yourself. Do you have any hobbies or interests that you could focus on? If you don't, it would be good to find some.

I always have a list of chores or projects that allow me to move to a different area and be busy in case I need to remove myself. Mine's go-to is stonewalling, so more often than not, it's going elsewhere to not be in his presence being actively ignored. Whether it's folding a load of laundry, working on a craft project, writing emails, scrubbing a shower surround, etc. Just refocusing your energy on something more productive sends a message to both your brain, and to him as well. It decentralizes his status in a way that's good for you.

Hope you get some peace.