Backup all data to GDrive ---> Hard reset.
DO NOT restore from that backup. Start with fresh new setups. Check for latest OS/firmware updates. Next, install all vital apps and test for 7 days. If there is no improvement or bugs still exist, sell it off...
The fingerprint issue, Smart Lock, Google Assistant (not to mention the VoLTE issue, but it's different story) still there before and after upgrading & reset.
Well, the issues are not frequently triggers though so I can still use the phone as normal, but it annoys me when it does especially during crucial times.
Might test again later once I received the new phone.