Found some 1000BASE-T #SFP <-> #RJ45 modules which work with the #uboot #tftpboot of this #Xikestor #Realtek #rtl93xx switch chip. And managed to figure out how to select a port other than the first one for tftpboot (just use the same weird, non-linear port numbers as the #DTS in #OpenWrt uses.) This now simplified my test setup quite a bit.