So far I've found this HBS article a very good start: https://hbr.org/2016/09/putting-products-into-services
This is also a good general advice:
https://www.pragmaticinstitute.com/resources/articles/product/transitioning-from-services-into-product-management/?trk=feed_main-feed-card_feed-article-content
I am looking for more in depth material. Case studies/frameworks/books/articles/blogs...
Feel free to look at my draft below, and let me know what’s missing.
Based on underwriting and accounting practices in that segment our company built core software libraries that embodied a common set of operations and transactions and best practices.
We would then customize these for the specific client. We would structure the contract so that it was hours billed plus a % like 20% that would be billed as licensing fee for the use of the libraries.
This would turn out best for both parties as we would get to bill multiple times to different clients for the work done once and they would save on the cost of building these libs from scratch.
We would of course factor in some free maintenance bug fixes etc but only in the core libs. Separate maintenance /support contract for application.
Some variation of this may work for you.
Some of the product companies are very speCIalized in what they do. There is no one size fits all.