I also wish it was 'one lesson from four years of building tools for ML'.
On a serious note, there is a book on Human-In-The-Loop ML by Robert Monarch, published just a few weeks ago [2], where concepts like "active learning" are elucidated. Also, Andrew Ng recently started 'Data-Centric AI' competition, focusing on improving the data but keeping the model fixed[3].
There seems to be a growing emphasis on data quality while models become commoditized and outsourced to 'ML as a service' (MLAAS) platforms. If I understood correctly humanloop project aspires to be 'all-in-one' MLAAS serving both the models/predictions but also taking care of data annotations, targeting the market currently served by e.g. Scale.AI and Salesforce Einstein.
[1] Bayesian Reasoning and Machine Learning http://web4.cs.ucl.ac.uk/staff/D.Barber/pmwiki/pmwiki.php?n=...
[2] Human-in-the-Loop Machine Learning https://www.manning.com/books/human-in-the-loop-machine-lear...
[3] https://https-deeplearning-ai.github.io/data-centric-comp/