I've supported 3 different models over the years with inference implemented in SQL. First one I inherited, loved it so much that I implemented it twice again. Amazingly fast for TBs of data and no waiting on other tech teams.

That tooling you're describing is definitely not there. Bigquery has BQML but it's very much in its infancy. I tend to do all the modeling in Python/R on sampled data and then deploy to SQL.