Get a log
Stuck on bootanimation? Give it about a minute and then press power + volume down while on the bootanimation to go right into TWRP
Bootloop? Wait until the bootloop happens and hold power + volume up to boot into TWRP but make sure you don't interrupt the bootloop with the power button press
This isn't foolproof. If you accidentally boot android instead of TWRP you'll have to redo this. The idea is to go into TWRP right after a crash
After a crash, the kernel checks the ram (ram on android doesn't get erased after a reboot) and recovers the kernel log in a directory called /sys/fs/pstore
You'll see console-ramoops-0, dmesg-ramoops-0 and pmsg-ramoops-0. Depending on the type of crash some of these files may not be present. You need to upload (preferably in private) at least the console-ramoops-0 file for helping diagnose this issue
Also it's recommended that you bootloop the device at least 3 times and collect 3 unique pstore logs as that helps sort out the issue even faster