Using fibre(-only) for links going to the outside does make sense (details depend on your location/ likeliness and severity of thunderstorms, distance to cover, etc. pp.). That way both sites remain 100% electrically isolated (don't forget electricity, water and heating, which may spoil your complete isolation <-- these also need to be properly installed).
I do echo the suggestion for using managed switches with SFP/ SFP+ ports on both ends of the link, probably not even more expensive than 'simple' media converters, but much more useful - and you don't even have to touch the routers/ APs that way (upgrade/ replace at your pace).
Up to 24 ports, you can probably find passively cooled switches (as long as you don't need PoE…), 48 port ones usually need (loud) active cooling, so two 24 port switches might fit better into a home environment than one 48 port switch. With PoE as a requirement, you probably need to go smaller once more, to remain passively cooled (so it does make sense to use a dedicated -small- switch for all the devices that do need PoE, at least as long as you don't need many of those).
Also take a look at your local used markets, sometimes these gs1900 series switches are sold pretty cheap.
About the gs1920 series, don't expect anything anytime soon - Support for Zyxel GS1920 series (GS1920-24HP) (and vendor support for them appears to be discontinued). https://svanheule.net/switches/models is a good source of information about managed switches and their support prospects for OpenWrt (if in doubt, ask about the 1-3 devices you filter down to).