Don't quit your job without your next source of income secured.
I guess this is the takeaway from a self preservation standpoint, but the alternative is worse. Spending months of your employer's time completely mentally checked out, taking sick time and vacation for interviews, pretending like you care in meetings, it's all too much. I watched that happen multiple times before I left, and it's really discourteous to the people you work with. It just seems unethical to me to remain somewhere once you've decided to leave.
Of course, it is more fun to work at a job you care about. But you are under no obligation to "care," and there's nothing wrong with keeping a job while you don't care until you find another one.
That being said, as difficult as it is to continue to give 100% leaving well is very important for building references that could help you in the future.
Not if you have any other kind of safety net, e.g. savings or frugal low cost lifestyle.
While I agree you shouldn't quit your job without having the next thing already in place, at least in my experience this is a very volatile industry and you can't really expect to keep a job for a long time anyways. The odds may be better at a bigger company, but it still happens. One of the largest employers in my city just laid off about 1100 people.
I've been lucky in that I've been able to quickly find work after these things happened before, but I look at it like I need to be prepared to potentially have to go without work for months at any time. Hopefully my next job will be one I can keep for several years, as the constant job hopping really isn't for me.
Not having a job when you are applying for new ones puts you on a back foot with salary negotiations, as you don't have anything to compare it to. However you do have a lot more time and energy to put into the interview process. Especially when people are looking for technical tests and you have limited spare time and energy after work.
Everything worked out well and it was the best decision I ever made.
Upon saying that, I knew that there were a lot of jobs available, and I was also willing to do any kind of job, not just software engineering. If push came to shove, I was willing to go back to working in construction or hospitality.
I also had a backup plan if all else failed, which was to fly back home and stay with my parents while I sorted my life out.
My advice would be to not quit your job without a plan and a plan B.
Either can be taken to an unhealthy extreme, but on balance the human body benefits more from physical exertion than the lack of it.
Sitting in an office desk for 30 years can't be good for you either, but if it's in a position to keep my brain active (even if my muscles aren't), I'd take that job a thousand times before going back to pure labor.
In fact, one of the most memorable parts of those summer jobs was creating a shoulder rest for the 25 lbs bottles to sit on, out of discarded plastic. I honestly can't remember whether it worked at all or not, but I definitely remember being far more interested designing and cutting this piece of plastic than carrying bottles of water.
Do you find programming jobs intellectually stimulating beyond junior level? Maybe I have really bad luck, but all that I seem to be able to find - and see my friends doing - are the computer equivalents of lowest-level construction work. I usually have to step into other people's competences (e.g. contributing UX or even domain-level ideas) to get anything interesting from them.
So are sedentary jobs. Obesity, heart disease and everything else that goes with them are no joke.
It's not exactly the same skillset, but you'll understand and appreciate sales and accounts receivable much more after a stint of doing that yourself.