Upscaling guest photos with a local model instead of an API

I run Knipsmig, a QR-code photo sharing service for weddings and parties. Guests scan a code and upload straight from the phone, no app. Most of those uploads are 12 MP and print fine. A meaningful slice are not: photobooth captures at 1080x810, WhatsApp forwards at 1600x1200, screenshots, old scans someone re-uploaded. Those end up in the printed...