This thread may be of some help: Support for RTL838x based managed switches
There are though some concerns whether support will continue atm.
I can’t speak to your specific questions on the Zyxel switches, but my Netgear GS308T on the RTL8380M has few issues afaict.
The TOH may also be some help: https://openwrt.org/toh/zyxel/gs1900-10hp