This is how I run my setup so I can run an x86 board from a chromebox as my router and have options from celeron to i7 CPU. Been this way for 5+ years and hasn't failed. Also get the benefit of AES-NI with some of the CPUs on CBs to offload VPNs tunnels.
What switch did you get? How do you get wireless?
Wireless I use a Unifi mesh system. Per the OP, I believe you wanted recommendation without the router providing WiFi. I use an HP 1810-24G which is a nice cheap enterprise grade fanless switch.
GL-MV1000 if you don't mind 3 ports, comes in a wireless and non-wireless flavor.
for 130 euros you can buy the Mikrotik https://mikrotik.com/product/crs305_1g_4s_in
It can run both the Mikrotik SwitchOS or their RouterOS, or OpenWRT
it has 4 SFP+ ports so you can use it with ethernet or fiber optics up to 10GB, so it's a future proof investment
Really? Where do I find the OpenWrt port to that device?
I loved the Brume (GL-MV1000), comes with OpenWRT firmware. Very easy to set up and can run both VPN (including Wireguard) and Adguard Home at the same time.
I returned it though, since it ran very, very hot.
The ER-X is a great choice and is well supported by OpenWrt. Downside is it's a little pricey given the dual core CPU isn't fast enough for SQM Cake shaping anywhere near 1gbps like the WRT32X or R7800 will get close to with OpenWrt. Another great option is like people said, the Raspberry Pi 4, but that takes some configuration and an additional switch to get working, you'll end up spending a similar amount of money.