If you are in EU, the nordic countries, USA or Canada, then teklager.se can ship APUs with OpenWrt installed. OpenWrt is included in the price when you buy the hardware. They are very nice people running their store as a side business, for the fun of it, so it would be easy to ask them for exactly what you want, for example an installation of latest snapshot. If you just order an OpenWrt installation you are likely to get the latest stable with EXT4. You can also chose OPNSense or pfSense.
The APUs are x64 boxes. APU2 can be fitted with two Wi-Fi radio expansion cards, while APU4/6 can only have one expansion card for Wi-Fi, but they can also have LTE. As far as I am aware Teklager don't sell any Wi-Fi 6 cards, only Wi-Fi 5. I believe there are threads on this forum where people runs cards from AsiaRF with Wi-Fi 6 in APUs.
The APUs are a fairly old design now, but they work fine and have plenty of RAM. They are not cheap.
The main drawback is that x86/EXT4 is a bit of an hassle to sysupgrade, see https://openwrt.org/docs/guide-user/installation/openwrt_x86#upgrading. You need backups of your config and an understanding of which kernel modules have been added for the Wi-Fi.
Edit: Ooops. I did not see that you are in the UK. It is not in their list of countries they ship to, but you could always check with them if they'll ship to you. They usually answer e-mails quickly.