Tasks either don't start or don't run as background processes
The issue seemed to stem from the fact that Caddy serves up PHP with fastcgi, instead of Apache or Nginx, and something to do with http requests used to start the task. AJAX requests (what my other functioning task plugins were using to kickoff) were unaffected.

Following a suggestion found here: https://stackoverflow.com/questions/11758334/fastcgi-finish-request-creates-hung-connection-when-open-session-exists

I've placed

session_write_close();
fastcgi_finish_request();


at the top of the runStep($step) function in each plugin's Task class and it resolved the issue!
Though, I'm not sure if either of the lines above have other implications.



Update
I should add, that from what I can tell, the above snippet does cause issues with tasks on Apache setups. (tasks will start yet never finish). Unless you are using Caddy Server in production, (you're probably not and using Valet for development) the snippet above should only be used as a temporary hack to get through your tasks.