Exactly, if you already have such a setup in place it's basically ordering a button online, e.g. https://sonoff.tech/en-eu/products/sonoff-zigbee-wireless-sw... then next day slapping it over your existing setup or sticking it on the wall then deciding what you actually want to do with it, e.g. phones notifications, sound on as feedback for mailman, logging events, etc then adjusting conditions e.g different sounds after sunset, dedicated audio if away but expect parcels, etc.

But honestly, even if you have nothing, if you know a bit what you are doing with IoT with HomeAssistant if you buy the right hardware (i.e. verified as working by others, e.g. the Sonoff ones typically) it's at most an afternoon of tinkering for a setup that will reliably work for years.

String breaks. Hole attracts insects. Bell is too quiet or too heavy.

Now we're in "under-engineering" territory. Also not good.

Yeah, will probably have to consider something like that.