Getting the thread back on topic (entropy pool slow to fill) - and /dev/random is blocking as noted by the slow start of WG by the original poster. It doesn’t just affect the AR300/MT300 devices on WG, it’s common in many other scenarios as well.
The 9531 has a HWRNG, but it’s questionable of the quality of that generator (see the OpenWRT thread mentioned below)
With devices like this, it’s going to be a bit of a challenge - as previously noted, haveged does fill the pool fairly fast (noting stated concerns about the quality of randomness generated by haveged) - OpenWRT has done some effort to improve performance there.
Good thread over on the OpenWRT forums - Randomness quality - Installing and Using OpenWrt - OpenWrt Forum
Included in that thread was options for a TRNG key (for the truly paranoid perhaps)
Another good read here on randomness in linux, including /dev/urandom…