I think the original poster's problem may have been because they included gdal-abi-x in their sudo apt-get install command. Apparently gdal-abi is a virtual package; not sure what that means but apt may have crapped out when it couldn't find gdal-abi. The original poster may have had more success if they left gdal-abi out and installed all the other ...
Finally it was an access rights problem, but not at folder level.
The ini file (/home/user/.local/share/QGIS/QGIS3/profiles/default/QGIS/QGIS3.ini) was read-only (I really don't know why).
I simply changed its access rights with chmod, and QGIS recovered its memory!
After you have verified the fingerprint you forgot to add the key to apt with:
gpg --export --armor 51F523511C7028C3 | sudo apt-key add -
Read the QGIS documentation. All the instructions are:
wget -O - https://qgis.org/downloads/qgis-2019.gpg.key | gpg --import
gpg --fingerprint 51F523511C7028C3
gpg --export --armor 51F523511C7028C3 | sudo apt-key add -...