I agree. There is too much shared state inside software dev work. I think the only time part time might work is if the person has worked for you full time for a while, is fairly established, is a low management overhead employee (low/no drama, no hand holding, self-directed, gets the point quickly when you ask/tell them in small amounts of description, doesn't need mentorship and and gets stuff done). But then what often happens with part time devs like this is their one foot out the door, so they will probably leave in a few months anyway.

In other professions where the shared state isn't as big, such as medicine or therapy, you see way more part time style work happen actively in the industry. Be it working for 2-4 clinics part time or just doing half hours so they can focus more on other things in their lives.

Another kind of role that I think works well part time is an advisor / consultant expert role where your hired to verify things are being done correctly in your sub field and are used to train others design wise as a 3rd party. Which is another sr / experienced type of role.

Another type of part time that might work is working on your own project as a solo dev in maintenance mode once your established. But also only lasts so long as competitors get ahead of you.

Part time not working well is something inherent to the job itself unfortunately.