This is a similar use case I had in mind when implementing the possibility of monitoring multiple Syncthing instances at once in Syncthing Tray. I have never really followed up on that because for me personally it didn’t seem that useful after all. But interesting to see that someone tries to fill that gap. I guess a web app like this is indeed the better fit for this use case anyway. It is nice that it also shows pending devices and folders.
Because you mentioned it on the README: It would still be nice if the UI was a bit more polished and more in-line with the official Syncthing UI. A little bit of CSS would already make a huge difference - you don’t need any fancy frameworks that will impact the loading time.
I had been looking for this kind of setup for a while, and finally settled with a custom api someone posted on r/selfhosted a few months ago for Homepage. I’ll still deploy this one on my RPi.