can somebody give me a link for the image file which I can use it for deploying RKE2 cluster

You can try focal image. https://cloud-images.ubuntu.com/focal/current/focal-server-cloudimg-amd64.img

I have issue with my console, that it can accept only tty1

I remember it's

sles

.

is it possible to generate the ISO by modifying the grub values?

You can specify any user with LEAP because it supports cloud-init. I typically use

opensuse

. You can customize an ISO with #elemental.

FYI default user with SLES is

sles

, with openSUSE Leap etc it's

opensuse

Checking my assumption that the username is configurable because it is an input field during cluster node pool creation, correct? That is, if the selected cloud image supports cloud-init then the username will be configured, if it does not then the field has no effect and it's still necessary to log in to the console or ssh with the image's distribution default privileged username.

Yes I believe that's correct

Are there well known advantages, e.g., integrations, to using LEAP or SLE or another SUSE-originated cloud image for a Harvester guest cluster node pool? I get they have different advantages like immutability or cloud-init support that are applicable anywhere that image is deployed, but I'm wondering about things like life cycle or monitoring as it pertains to Harvester specifically.

Harvester doesn't care what the OS is so it just comes down to choice and/or reasons.