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I assume you tried using Joplin as a to-do list app? IIRC it worked, but was too fiddly with the square brackets.
Tasks.org is an open-source to-do list app working with multiple providers. On first glance, CalDAV and EteSync might be something for you (ignore DAVx⁵ as it’s a worse CalDAV).
As for me I’m using CalDAV my mail provider mailbox.org is providing me together with Tasks.org on Android and Thunderbird on desktop.
I would say, it’s them caring about the product and their needs, rather than the underlying stack.