Logic Board Replacement is too expensive/not worth it - suggestions on where to sell Macbook for parts

I would be concerned about your data. While a 2019 system has a T2 security chip so that all data on the SSD is naturally encrypted at the hardware level (so the data is not accessible), but my concern would be if someone repaired the Logic Board and was able to boot your Mac and log into it. If they can log into your Mac (or guess your login password -- so many people have simple passwords), then they would have access to your data. Just make sure you sell or recycle it with a trust worthy vendor. I'm a bit paranoid about personal data and don't trust the people out there (most are probably Ok, but you never know who will end up with the Logic Board).

I have no idea. Perhaps another contributor will have some suggestions.

This is one of the downsides of not having a removable SSD. At least the older T1 Macs could be connected to a special device to access the SSD to wipe it even if the Logic Board was bad, but this is not an option with a T2 Mac.

Edit: I should clarify your data is safe as long as the Logic Board is not repaired since the data is hardware encrypted. If the board is repaired, then the data is only as secure as your password to the admin user account(s).