The standard route is to decrypt on boot. It happens after GRUB but before your display manager starts. IDK if there even is a setup that has you “decrypt on login”. Thats sounds like your display manager (sddm for KDE) is decrypting system which is not possible IMO.
Unless your laptop somehow has multiple drives you’ll want to use the “LVM on LUKS” configuration. 1 small partition for /boot. The rest gets LUKS encrypted, and an LVM group is put on the LUKS container. Or you could replace LVM with btrfs.
This will require wiping your system and reinstalling so you have some reading to do.
The arch-install script in the live iso has options for full disk encryption.
If you suspend to RAM your system will stay unencrypted, because your ram is not encrypted. if you suspend to disk (aka hibernate) your system will be encrypted. You go through the boot loader when waking from hibernation but it just drops you off where you left off.
You need a swapfile for hibernation so make sure its inside the LUKS container.