Available only in snapshot: https://downloads.openwrt.org/snapshots/targets/realtek/generic/openwrt-realtek-generic-zyxel_gs1900-8-initramfs-kernel.bin
No... It's a new target. It didn't exist in 2019. It was barely there in february 2021. The absolute oldest you can expect to run is 21.02. And that is only on a limited number of models and with severe usability limitations (no fw_setsys by default for example)
Configure a tagged vlan 100 on the other end of the link connected to port 1. I.e. on the system you use for installation and initial configuration of the switch. The switch is already configured like this, so you have to match that config to be able to talk to it.
Networking is not enabled by default in the bootloader and must be turned on with
rtk network on
before you can tftp anything. So you need console for that.