Well my wifi is still no good. With other macbook's and an iPhone 5 working fine, PB 2 messed mine up and PB3 has not changed that.
EDIT: Ok, so I found this article about changing my DNS, even though I had tried this before.
https://www.igeeksblog.com/how-to-change-dns-on-iphone-ipad/
The only thing I didn't do last time was to flush my old DNS settings by using airplane mode, etc.
Anyway, at the moment with my very quick 5 min test, this appears to be working nicely and my 5ghz wifi is back to it's best. Still very weird as every other Macbook or iPhone is using automatic settings and is still ok, BUT the iPhone 5 is not on iOS 12.
I will update this post with further info once I've used my phone today.