The Opal/Beryl/Slate Plus are just a shade smaller than the AXT. You can see the Beryl and Opal are not much of an improvement in OpenVPN, and even the Slate Plus isn’t that much because OpenVPN is single threaded. WG, which is multithreaded, takes advantage of not only the higher clock speed but the multiple cores of the Slate Plus.
You might experiment with WG if you haven’t, and splitting the radios so that the WISP connection is on one band and the LAN on another, to see if your throughput increases. Also, a wired connection helps a ton.