[Update]
I am still looking for a solution, but meanwhile I gave up with Kernsave https://www.kernsafe.com/download/macos-iscsi-initiator.aspx and the above mentioned OpenSource Initiator. Kernsave works nice while you are using it. Also the limitation to only one connection would be not a problem. It's more the fact that you can't create a setup where the connection is done automatic to ensure Time Machine find the volume.
The OpenSource Initiator is nice to handle from CLI, but has difficulties with Apple's SIP ( it works when SIP is off and it can't store preferences when SIP is on) . Also the developer has mentioned that Apple is moving away from KEXT to driverkit and it seems that the SCSI is only available with Big Sur.
So I am looking forward to the upcoming SCSI driverkit and mybe a iSCSI Initiator is already bundled with the OS ...ok, the hope is dying at least :D