I would not blindly copy over /etc if you are not copying over the rest of the system files. What you need will be recreated when you reinstall your packages. Files in /etc you have explicitly modified (if you have a customized pacman.conf or sshd_config or something) you can copy over piecemeal. For most folks, going back after the fact and adjusting any configs that were forgotten will be trivial.
Most of your user-specific configuration is stored in dot files in the home folder. After you have finished installing all your packages, if you copy over the home folder you will probably be pretty close to where you left off--even your browser history, etc should be in there.
Are you giving up on the disk migration? It seems like something that should be possible--just a little tricky, because you are changing from a /dev/sda to a /dev/nvme0n1. I would expect /etc/fstab may need to be revised, and reinstalling Grub was not a bad call.
I had tried to do the same and after like 2 hours of tinkering and even asking my friend who has more experience with Linux...we both gave up and i just backed up what i wanted and did a clean install.