It is possible. I have successfully migrated all of the data from Google Timeline to self-hosted owntracks. Quick and dirty conversion script is available here - https://gist.github.com/skrobul/bd1f64309123c34bf5cfcad144e6...

I have not looked into deleting data from Google yet as my free time is taken up by migrating from other Google not-really-forever-"free" services.