> But there is still no way to use ndb from outside App Engine standard, right?

Shameless plug, but for Go, there is an unofficial third-party (Chromium team) datastore package that feature near-seamless AppEngine standard/flex datastore support: https://chromium.googlesource.com/infra/luci/gae/+/master

It currently couples to any hosted memcached service, but should be forward-compatible with AppEngine if/when they release a Flex-accessible cloud version of their hosted memcache.

"luci/gae" also features a fully-compliant in-memory datastore mock for 1000x+ faster datastore testing and uses a memcache implementation based on "qedus/nds" which fixes several consistency issues present in "ndb".