After many weeks of frustration, I have finally found a solution to the issues with syncing tasks between my MacBook Pro (OS Mojave 10.14.6) and my iPhone (iOS13 without upgraded Apple Reminders lists). Ever since updating my phone to iOS13, GoodTask was terribly slow and tasks wouldn't sync without lots of manual refreshes of both apps on both devices. I was often unable to mark tasks as completed on my phone. GoodTask worked pretty well on my laptop so I switched to doing most of my task organizing and creation on there. But then all the changes, completions, reschedulings, and new tasks I did on my laptop would take forever to sync to my phone. Eventually I figured out that the problem was with iCloud/Apple Reminders and not with GoodTask. I tried deleting all completed tasks from within the GoodTask app on my iPhone (as suggested in the update notes), but it didn't help and I don't think anything was deleted. I couldn't figure out how to bulk delete my thousands of completed tasks...until now!
The Solution: Go to icloud.com, log in, and open reminders there. Select a list and then click "1,234 Completed" under the list name in the right panel. Then click "Clear Completed" on the right. I got a lot of errors that said something like "unable to delete this task" every time I did this for each list, but the tasks would still get deleted. It took a while for me to delete everything from every list, but it worked! Now GoodTask is fast again, Apple Reminders stay synced between my phone and laptop, and the iCloud-GoodTask sync works perfectly again.
I also just read that you can select multiple/all completed reminders in a list in the Apple Reminders app and bulk delete them there. I just tried it on my laptop and it worked. I think I had tried that a few weeks ago and it didn't work when I had thousands of completed tasks.
Hope this helps others who are having sync issues!