Looking at a OpenWrt router build

Budget (including currency): n/a

Country: UK

Games, programs or workloads that it will be used for: openwrt wifi routing and as a hub for a wifi star network topology. Id like a wifi router that can scale with me currently i only have 40mbs adsl internet but i will be upgrading soon to fibre. id also like it to have 2 sfp+ ports (one for wan one for lan) 1 rj45 (for my current speeds) and some mpcie for wifi cards. I'm ok having used parts.

so I've found a few things here paired with this, an m.2 boot drive, some ram and 2 mpcie wifi modules with full bandwidth g/n/ac/ax. I'm wondering if this will work or if anyone has experience with this. after a lot of googling it seems not many people know this exists and with this being rather expensive I just want to make sure it will play nice. Thanks for any help

Get a small pc with one or two pcie slots, buy a used sfp+ card, and put in the pc.

Add router or AP for the wifi functionality.

Even fibre doesn't imply a 10 GBit/s contract by itself, if you don't need >1GBit/s for the forseeable future (18 months to 3 years), don't consider that yet - prices are still prohibitive.

If you're UK based, the Belkin rt3200 (often on sale for under 80 GBP) might be a decent stopgap measure until you really need to break the 1 GBit/s barrier. The RPi4 based solutions would also still apply, should you prefer that.

Yes, I do think x86_64 is a good choice for a router (using one myself), especially above ~500 MBit/s, but 10 GBit/s (especially on the WAN router, before you have your LAN upgraded) is still a markup I wouldn't consider at this point - unless you need it now/ tomorrow.

Get a separate box for firewall/routing, this will make your life much easier.
Go for a business oriented desktop, like the Fujitsu Esprimo D738 (or something similar) and grab low-profile network cards of choice.
As for wifi, I'd advice you to go for 11ac hardware as it's mature and work relatively well. ipq40xx or Mediatek based (the later variants) seems to work well in my experience. Linksys MR8300 or Totolink A8000RU are a few examples of these platforms.