tarsnap 1.0.40 (on 2022-02-10) added --resume-extract, which skips extracting files which are already on the disk. This means that if you restore half of your files, then it's interrupted (another power outage, cat on the power bar, whatever), you can pick up where you left off.
That said, this functionality might already be part of redsnapper. I'm not familiar with ruby, so a quick glance at the code didn't clear it up for me.