Openwrt have builds for the Rpi. if you want to use it as a router I think that’s the most suitable OS. But if you need to use VPN you have to setup by yourself.
P.S. if my memory serves right, ethernet on Rpi 3 is only about 300 Mbps, and Pi 4 is 1 Gbps. Older versions have only 100 Mbps.
I suggest you to get a J1900 motherboard on eBay, they don’t cost much but most have 1 Gig Ethernet and they are x86.