Monolithic router vs barebone/mini PC router

Hello forum people,
I'm looking for a router that is is not dependent on a specific manufacturer, flexible, energy-efficient, has support for Enocean protocols and is upgradeable. In order to achieve this objective, would it be a better idea to use a mini or barebone PC with additional hardware or a normal monolithic device as router? In other words: Can you build a router from scratch that has more capabilities than a TPLINK ARCVR2800 or a Fritzbox 7590?

They're both A / VDSL modems, which is hard to recreate ...

Thanks. Another question: Does OpenWRT allow me to run other Linux software in parallel so that I can unite routing functionality, web and VPN server and smart home gateway under one roof or would it be a better idea to use a lightweight version of a normal Linux distro for this task?

It can do all of it, but I'm not sure about the smart home gateway, since I don't really know what it means.

I'd still run a regular Linux system instead, though.

If you're planning to do all that, I'd much rather recommend running a server-distro and then OpenWrt in a container on top of it, or setting up a virtual-machine hypervisor like e.g. Proxmox VE or XCP-NG and running OpenWrt in a VM.

Routing, VPN and such traffic-related tasks are all a-okay to run on OpenWrt, but I wouldn't be running proper web-servers on it, but instead in separate VMs or containers for security and the ease of keeping them up-to-date at all times. OpenWrt isn't really the right tool for running such things on.

I think there was some way to install Homeassistant in OpenWrt.

Thanks for your answers. A smart home gateway is a device that runs a scheduling program which sends telegrams to switches in order to control a lamp, a roller shutter etc. There are different home automatisation standards. I use devices that are manufactured according to the Enocean standard, because they are optimised for low-energy consumption. I don't even know if it is a good idea to unite router, web server and home automatisation gateway in one device, but it seems to be an obvious solution if you have a small website that doesn't receive many requests.

I would do it too, my only concern is whether the functionality exists in openwrt.

I found out that software exists that allows you to control different home automatisation standards (ZigBee, WLAN, Enocean, Z-Wave, Bluetooth...) within one program. The best tools are openHAB and Home Assistant. Is it necessary to choose a device with more than one Ethernet port or is an extra Ethernet switch sufficient? I'm asking because mini PCs with multiple Ethernet ports are rare.

mini PC will require a multi NIC card for multiple ethernet.
Homeassistant has the best support/community and compatibility.

No, they aren't. Not even close. You can find tons of such on e.g. eBay, Amazon or Aliexpress, if you search for Pentium N4200, Celeron J4115 or Pentium N6005. I, myself, have an Odroid H2+ with a Celeron J4115 and an UP Squared with Pentium N4200.

The H2+ is unfortunately discontinued at the moment, but Odroid will almost certainly sooner or later produce something similar with a newer SoC.

I've been running OpenHab for years, myself. It's not the world's easiest thing to set up, but it's very capable.

Not necessarily. One can always use VLANs or one can use e.g. MACVLAN to give containers and/VMs each their own, virtual NIC over a single physical NIC. It all depends on one's specific needs.

I could also connect a single Ethernet interface to an Ethernet switch if the NIC is at least a 1 Gigabit card, because the bottleneck is in most cases the connection to the ISP. Are there any Ethernet NIC manufacturers that I should avoid when using a Linux machine?