Even a PCIe adapter will generally have lower performance than a purpose built AP... that comes down to the antenna system more than anything else.
Define 'cheap'? Once you add in the cost of the Pi5 (or a mini PC) plus the 6E card, you're already looking at more than the cost of a proper AP. For example, the Ubiquiti U6 Enterprise is $279 USD -- that has a 2.5G ethernet link and can support up to 600 simultaneous client devices (at least on paper).
There aren't many 6E routers supported by OpenWrt -- this thread covers it (although it's possible it's a bit dated).
The fact is that the every-day typical performance difference between wifi6 and 6e is often barely noticeable for most typical use cases. In fact, many people don't see a huge difference between wifi5 and wifi6 -- probably the most significant difference is the handling of higher client density. Therefore, you may be better off with a standard wifi6 device instead of bothering with 6E.
Also, pretty much any wifi router can be made into a dumb AP provided that you can set the IP address and turn off the DHCP server. Obviously a purpose built AP (that is not designed/advertised as a wifi router) has that already in mind.
If you want to run OpenWrt specifically, you'll find many more choices in the wifi6 range.