You're actually using systemd-boot, not GRUB.
Solus sometimes names its bootloader file grubx64.efi for compatibility reasons, but it's not the actual GRUB bootloader. The output of bootctl status shows you're using systemd-boot (e.g. the .conf entries in /boot/loader/entries/ and the "Boot Loader Specification Type #1").
If you're seeing the boot menu for a few seconds at startup, then your loader.conf file probably includes a timeout value.
You can check it by running: cat /boot/loader/loader.conf
. Look for a line like timeout 5.
If it's not there, the default timeout is 0, and the menu won't be shown at all.