One way of thinking about this is that you’re charging less for doing less work. Maybe that’s what you want (eg less stressful days). Another way to see it is that if you could work for some places you like less for £X per day, you could live off £500 per day and donate £(X-500) to charity (this is basically the argument that was popular 10 years or so ago that ambitious young people wanting to ‘do good’ should go and work in tech/finance to make a lot of money to give to charity instead of working for charities directly). So in some sense you’re choosing to instead donate £(X-500) per day to the nonprofits you work for. I guess if you like what they do then maybe it’s a reasonable choice for you.