I would personally be totally fine with a backup solution which is orders of magnitude slower and several times as expensive in the rare restore case if it mean

I would personally be totally fine with a backup solution which is orders of magnitude slower and several times as expensive in the rare restore case if it meant that the typical "write only for long periods of time" scenario was significantly cheaper. My personal backups are not something I'd ever need to restore in a hurry. The one big downside to the "cheap backup, expensive restore" approach is that it discourages testing your restore.