This is potentially more hassle (than using public DNS) as you have to get your CA certs onto every device. However it may be suitable depending on the situation.
I think you've hit the nail on the head so to speak....it's just too small/custom a thing for anyone to have built a dedicated tool it seems. In the end I am looking at using my file manager (nautlius) to automatically run a custom exiftool/bash script on chosen files so I can just click and rename/fix metadata etc as I browse through the files. Probably good enough for now.
So git-annex should let you just pull down the files you want to work on, make your changes, then push them back upstream. No need to continuously sync entire collection. Requires some git knowledge and wading through git-annex docs but the walkthrough is a good place for an overview: https://git-annex.branchable.com/walkthrough/
I think you can also configure the system to take action when it reaches the lowest level with e.g.
# The action to take when "TimeAction" or "PercentageAction" above has been
# reached for the batteries (UPS or laptop batteries) supplying the computer
CriticalPowerAction=PowerOff
However I don't know how to get these GNOME "Power" notifications to play an audible sound (without turning on notification sounds for ALL notifications). The best I could find is this: David Bazile / gaudible · GitLab
Home Assistant can do shared lists and (I've not used them) but has some recipe add-ons. There are apps for android and iOS. It can also take care of managing the dynamic IP. Then if you want to explore home automation in future you're ready to go.
Not on Firefox, some site functionality is disabled: https://medium.com/@leonardodna/the-ultimate-newbie-guide-for-self-signed-certificates-d81aa3b9987b