Watch R815U - not turning on, charger blinking red, no signs of life

I have had this watch since new but haven't used it in a year or two. In the past I've had poor battery life with it and eventually stopped using it. I went to use it for the first time in at least a year, maybe even two years, and tried to turn it on. It started to turn on as normal, showing the usual Samsung boot logo, so I put my sleeve down over it and left for work. After getting to work, only a 10 minute drive, I went to connect it to my phone but it was completely unresponsive, black screen, no phone connection, no signs of life at all. I assumed the battery was flat given that it's had poor battery life since before I stopped using it, and it's been on the charger the entire 1+ years I haven't used it. I ordered a new battery from iFixit and have just replaced it. I do not remember its charging behavior before taking it apart, I took it apart before the battery arrived to check for loose connections, etc. The watch, with the new battery, still shows no signs of life, causes the charger to show solid red for about one to two minutes, then blinks red. I have cleaned the internal contacts with isopropyl, reseated connections inside, and tried a different USB AC adapter with no improvement. The metal shield on the motherboard definitely gets slightly warm to the touch when attempting to interact with the watch, but still no display output, sound, vibration, etc. Also, the charger is the original Samsung watch charger included with the watch, and the watch has otherwise never had any issues, damage, or been opened for any reason in the past. Any help appreciated.