Unwise to use the SD card on a spare rPi as remote backup storage?

I'm slowly trying to get my Immich instance backed up with a proper 3-2-1 backup strategy despite my near-$0 budget. I have a friend who is willing to keep a backup at their place, and I have a spare Raspberry Pi 3B+ that I can use for this, connecting via tailscale and using either backrest or borgmatic for backups. I'm wondering: can I just back up to the micro sd card in it, or do I need to spring for an external ssd? It looks like the TBW ratings for good sd cards aren't too far off standard SSDs, but my gut still feels like sd cards are unreliable.

Edit: to clarify, I already have two copies (original and backup) at my home, this would just be to get a third copy off site.

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