I doubt not getting paid enough is the problem. although a contributing factor. it doesn't necessarily leads to bitterness, unless other factors come into play.

Such as the scenario of providing feedback on how certain technical choices made by (supposedly technical) managers, in contradiction with senior architects and most senior developers, are going to cost a lot to reverse down the road, then when that's precisely what happens, and being explained the company isn't in a position to increase your salary, that manager gets promoted anyway, and 3 new hires joins in to help reverse the misguided technical direction, but of course new hires have no clue how anything was put together, so end up asking the long timers some help, help that is pretty much doing 80% of work to meet deadlines, and then of course the newer hires' manager ensures the credit goes to his new reports, so that he can at least give those a rise and retain them. and also control them since they still little exposure to the entire stack.

the bitterness is usually not about the money, it's about how a few bad apples know how to play power games and ultimately ruins all the hard work of long ago gone managers who somehow made meritocracy at the core of the company. Those managers weren't very technical either, but they worked more ethically and shined at their role.

note: this isn't my own anecdotal story. it's a theme I've started seeing more and more in the last 5y or so, plaguing mid size companies the most, those that reached a level of maturity.

“I had proposed a design two years ago for this. I’m happy to share those documents and explain why, this approach should still be viable. Good luck implementing it!

I am not available for the project unfortunately.”

Don’t be a chump.

Not working is a perfectly valid option for dealing with burnout. Finally start that business, do some consulting, put that savings to use as a cushion instead of just a high score. Jobs will be there when you're done.

COBRA depletes the cushion very quickly. I feel like if your employer made you unhealthy, they should pay to make you healthy. That's why I recommend avoiding the rage quit plan. If you live in a country with government-provided health coverage, the equation changes a lot, however.

the UK unfortunately only seems to provide long term sickness support, which is a slim weekly allowance. they don't hesitate to take away 33% off gross salary in tax and other contributions, but no safety net.