Thank you for taking the time to answer throughly! I noted your advice and chunked up my goals into "mini-projects", once I have all the configurations set and all devices connected to the new router. I did check what I bought is a router, not a switch (I find the naming of the device acting as the gateway between the LAN and WAN to be a bit ambigous: switch, router, gateway...).
As for the IDS capability, this is something that would be done by a raspberry pi being fed packets from the router. I don't know if I will ever undertake that task, but I keep it in mind if I'll feel adventorous 🙃
(for those wondering: Linux Magazine #279 has a guide on how to accomplish this with a Fritz!Box 7583).
Thank you for taking the time to write this! Well, first stage of my project (getting openwrt my router) has gone according to plan, and now to strive for the next objective 😏