Every tool you adopt is a bet that it will still suit you in five years. The bet is much safer when leaving is cheap. So before you move your tasks into OpenTodo, here is how you would move them out.
One file, whenever you like
Every person can download a complete export of their data at any time: projects, sections, tasks and subtasks, labels, reminders and saved views, including completed tasks and the trash, in one JSON file.
A documented format
The export format is described in the documentation, field by field, so another tool, or a short script, can read it. Coming the other way, a CSV file from a spreadsheet imports too.
An open API
If you would rather pull your data out programmatically, the REST API reads everything the app can see, with tokens you control.
And the source code
OpenTodo is free software under the AGPL. Even if this project stopped tomorrow, the version you run would keep running, and anyone could continue it.
I would like you to stay because OpenTodo is good, not because leaving is hard.