Cron timeouts at 60 seconds, but all the related timeouts are set higher
Solved. It was the timeout of the proxy connection to PHP-FPM. It involves going down a rabbit-hole of Apache values and defaults so strap in.

As per ProxyPass the proxy default of connectiontimeout is timeout.

The default value of timeout is ProxyTimeout

The default of ProxyTimeout is the core Apache TimeOut

The default of TimeOut is 60. So there you go.

I set a value of TimeOut 180 in my Virtualhost for the site in question and the import went through.

Now just to wrangle all these values back down to a sensible level now that I know the culprits.