Being amazing at negotiating is a salary multiplier regardless of whether the job has meaning or is part-time... For example, as an absolutely amazing negotiator, you might get yourself a $1m full-time meaningless job or $100k part-time meaningful job. $100k is not a small amount, but you're still leaving $900k on the table.

Plus, let's face it, most devs are fungible commodities and don't have much bargaining power on top of the prevailing market rate. For devs, most bargaining power comes from domain expertise, but d.e. severely limits the scope of the job search - i.e. you may be limited to only a handful of companies worldwide which would be interesting in paying extra for that expertise. Chances that any of them do "meaningful work" AND allow part-time are close to zero.