Last time I was looking (pre covid) it was kinda hard to find readily available hardware to talk on 5.9GHz where there V2X 802.11p works. That is probably easier now that "Wi-Fi 6" uses that same chunk of spectrum as 802.11p. I'm guessing there is now (or soon will be) a Wi-Fi 6 usb (or perhaps PCI or M2/nVME) adaptor with enough flex in it's firmware that it can be convinced to talk 802.11p. Then you'd be talking a RasPi and a WiFi adaptor to start hacking on this.

There's a SDR openWiFi project that lets you build 5.8GHz WiFi using one of a list of SDR boards - it'd probably be fairly easy to tweak that into doing 802.11p, but now you're talking many hundreds of dollars worth of hardware - a bit outside FlipperZero/RasPi+dongle pricing.