I would recommend you use a migration tool like Sharegate or something similar. Migration tools actually migrate the site - they make changes on the back end to make sure that links and stuff still work. A backup/restore usually requires restoring to the same path - which won't work for you if you're going to SP 2019. It also won't make any of the backend changes. So even if you get the restore to work, you may find that the site itself doesn't function.
This is one of those things where if you want to do it, it's best to do it right even if that means putting some money into it. Any shortcuts you try to take will only cost more in the long run with downtime, rework and possibly remigration.