Looks like you've already had some help fixing your system. I'd recommend reading the arch wiki pages on encryption. Even if you're not running arch, the wiki is a good source of up-to-date information, and contains many practical tips.
This section about setting up your /etc/crypttab to unlock a non-root filesystem at boot might be useful. Even if you don't want to unlock at boot, it explains how crypttab works: https://wiki.archlinux.org/title/Dm-crypt/System_configuration#crypttab.
There's another useful section if you want to decrypt on demand after boot: https://wiki.archlinux.org/title/Dm-crypt/System_configuration#Mounting_on_demand
Lastly, if you're experimenting with fstab entries for a non-root device, consider setting nofail so that if you screw anything up the system still boots. This section is somewhat relevant: https://wiki.archlinux.org/title/Fstab#External_devices
Basically, you need both a crypttab entry and an fstab entry to open and then mount the device.
Based on your systemd output, it looks like the system is taking a long while to decrypt your drive. Is it a spinning disk, or an SSD?
I'm not sure if the PC repair shop specifically checked your drive, but it might be worth swapping out for another. Or maybe run some speed tests and/or diagnostics to see if there's something funky going on.
You could also try an unencrypted install to see if the problem persists.