I was also looking for a screenshot in the readme. I somehow missed the link to the demo until it was posted at the top of this thread.

Neat project. Thank you for sharing.

You might benefit from taking a look at https://www.openseizuredetector.org.uk/

It supports cheap smart watches like the PineTime and BangleJS (I would not recommend a Garmin device since Garmin likes to break their API). You might not be able to use the HRM due to furr being in the way but you can hopefully still use the accelerometer data to determine if Yuri is having a seizure. You might need to change thresholds for the algorithms based on accelerometer data to better match cat seizures. OSD is developeded mainly with humans as the intended sensor wearers.

This is amazing and very very promising. I was looking for something like PineTime but apparently my google skills weren't good enough.. Thanks a lot for the help!

I personally have gotten better stability out of a BangleJS with OSD than I got with a PineTime. OSD isn't perfect but the source is open and it wasn't too difficult to build locally (you mention you are an iOS dev, and OSD is a Java Android app, I bet there are still many similarities).

I'd wager that you can quite rapidly adapt OSD to become what you need. Maybe even upstream a nice little species selector feature perhaps?

The main OSD maintainer is a rather nice fellow. I think he'd be delighted to hear that someone found OSD and had use of it.