Sounds to me like either a software issue or an issue with the Motherboard. Unsure of what country you’re in, but if you are in the USA or Canada, and there is a uBreakiFix store near you, you can always go in and see if they can reflash the samsung factory software. There are 2 options, a full factory reset, or a reflash of the update software. The update may or may not fix your issue, and if it fails there is a small chance it would require a factory reset after, but the update function itself does not erase user data. So if all else fails and you’ve been unable to recover the data any other way, this may be a way to -possibly- fix the device. But, as I said, there are some dangers, although unlikely to occur.
Another thing you can try, let it keep rebooting and draining the battery and then when it doesn’t have power to turn on, let it sit for a couple of weeks. This’ll be the closest thing you can get to actually unplugging the battery to do a full power cycle, though it does take some time. Basically you would be giving the board enough time to allow the electricity it’s built up to dissipate, clearing the board of electricity. Again, unlikely to work, though that’s my best guess.