I use kSync but really don't like Open Tasks.
Tasks.org works with NextCloud, it's both available on PlayStore and F-Droid... I'd like to use it to sync my kTasks with any app I want to use regradless the OS (Linux, Win, Android for my part).
I'm pretty sure it's possible but I never found any documentation about it - and that's the issue : not being able to use any open source service to sync with an open source based provider
It works with https://play.google.com/store/apps/details?id=com.infomaniak.sync and open tasks https://play.google.com/store/apps/details?id=org.dmfs.tasks
I think there's a guide on their KnowledgeBase.