Running restic from the phone doesn’t actually sound that bad, but it does sound a bit unwieldy, especially if I want to add custom flags to restic.
Yeah, some amount of scripting would be useful if running restic using tmux or similar, and I’m not sure how to solve the problem of scheduling.
This forum thread might start you on the right path, there’s an android app linked towards the bottom of the discussion that looks like it might work for your use case, although I’ve not used it myself (I personally use the Syncthing approach mentioned in that thread).
Also I’ve already made my first full snapshot of the phone memory using a temporary mirror on my PC, so having to delete that to start from scratch would only add to the inconveniences.
No need to delete it. The source data is the same, so while restic will have to re-scan everything during your first backup run against the new path to the data (either mounted MTP, or otherwise), it will identify the data all exists in the repository already, so only write some new metadata. You can delete the old snapshot for neatness once done, or just leave it be.