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.
No. I've been doing part time work in tech for almost 20 years, and have done part time work for many "meaningful" jobs: working for scientists, contracting for non-profits, building better ed tech systems, working for hospital fundraising foundations, school boards, etc. There's a lot more to negotiation than money. In all of those I've been able to charge decent market rates, and work part time.
I'd say helping the forest service for environment canada collect forest fire data is meaningful, as is helping track underwater noise to reduce marine mammal damage.
It is absolutely a seller's market for developers right now. If you can't find part-time work in a meaningful field, it means you don't know how to look for work and ask for what you want, full stop.