This looks great. I've been trying to hack something similar on top of Silverbullet but having a dedicated app potentially using the same Markdown folder sounds much better. Will the desktop and web apps be available on your Github?
I've been mulling over many tablet options recently with a goal to reduce the number of devices I travel with (terminal, email, playing HW accelerated video, reading comics). I considered everything from an iPad through a Surface with Linux and all the way to more exotic options like https://junocomputers.com/product/juno-tab-2/ or https://www.fydetabduo.com/ but they all lacked something. I was close to going with a Google Pixel with Graphene OS but Google's atrocious pricing in Europe made me reconsider. This one finally made me pull the trigger, mostly because it is Linux-first and came with no "This is a beta product" or "generally works, but..." asterisks. Fingers crossed that it delivers.
I'm in the self-hosted camp as well with two Synology NAS boxes in two different countries that all of the family's computers synchronize to using Syncthing. Each of the boxes runs a local backup to an external drive nightly and one of them also runs a backup of a subset of (really important) folders to Backblaze. This uses Synology's proprietary backup tool (HyperBackup) which I intend to replace with an open source solution (most likely rclone but I'd be interested in suggestions). As an additional measure I rotate the external drives in one location weekly. So far it proved quite reliable when switching machines (in combination with git-managed dotfiles and stow) and accessing data on demand. I also make a full image of my laptop on an external drive more or less once a month to enable quick restore in case my OS gets hosed. One problem I still only semi-solved is synchronizing iPhones. Right now we just synchronize photos using Synology's DS File and I do an iTunes backup using a Windows VM which is clunky. High hopes for libimobiledevice here but I've had no time to properly research it yet.
All of the above requires some work but it's fun more than a nuisance. Probably not that great of a solution when I'm no longer around though.
The touchpad problems are well documented, e.g. https://bugzilla.kernel.org/show_bug.cgi?id=197683. If your XPS is still under warranty you can request touchpad replacement referring to the thread I linked to above. I'm just scheduling mine, hope it fixes the problem.
There is no concept of a single "Master". The folder types are "Send & Receive" and "Send only" and the latter will not accept changes from any other client. I still agree it's not the best solution for a network you don't control and that might have a malicious actor in it.