Don't buy a Raspberry Pi for OpenWRT.

The MicroSD card storage is unreliable and USB boot from an external hard drive was actually worse in my experience, due to the flakey USB Attached SCSI Protocol kernel support (which will vary widely, based on the bridging chip used in your USB-SATA adapter).

As a rule of thumb, when it comes to devices that need 24/7/365 uptime, avoid hot-pluggable devices, like USB storage or networking. Hotplug support and reliability simply do not work well together.

I use a TP-Link ER605 v2.

It's pretty easy to flash with OpenWRT and has good compatibility (the original TP link firmware is actually just a fork of OpenWRT).

Comes with 5 gigabit ethernet ports built-in (so no need to purchase a separate switch, unless you have many wired devices) and costs only £52 on Amazon.

Not had any issues with it so far, beyond syslog messages filling up the limited 128MB storage. A USB pen drive fixed that quite easily.

I run multiple applications on it such as Nginx, Zerotier, Adblock, SQM, PBR, Cloudflare Tunnel etc. and the average load is about 40%.

I'll stick with a Flint 2.

Articles like this are cool. If your router is still plenty fast (which is highly likely), but is run out of software security updates, or you don't trust its OS regarding privacy, or you want features that OpenWrt has, try OpenWrt.
Ideally would be a fully open source router, including open firmware, down to RAM initialization code (code for DDR3 RAM exists), and OpenWrt installed by default, but because of the economy of scale, such a router is going to cost more and price is simply the lowest common denominator for many.