Remarkable

RCU can also upload "splash screens" (including the sleep screen, power-off screen, battery dead, overheated, and others). It uploads the files to a part of the tablet which doesn't get erased by the OS upgrade process, and "links" them to where the reMarkable software expects to find them. The first time RCU connects to a tablet after an OS upgrade, it will detect that there are splash screen images which aren't "linked", and will offer to "re-link" them automatically.

RCU can also be used to upload custom templates, and it also uploads them to the document storage filesystem and "links" them to the directory where the reMarkable software looks for them, and will offer to automatically "re-link" them after an OS upgrade. Plus it can upload and download documents, either as a PDF (the same way the built-in web interface does it) or as .rmn files, which can later be uploaded back to a reMarkable tablet with the pen strokes still edit-able.

RCU isn't free, but it isn't expensive. I paid $12 for it, and I'll be happy to renew it after a year (although I would also be happy to pay a higher price for "updates for life").