Since we are not implementing UI 6 Definition Builders ourselves in the end, let's give a pointer towards Merkle's library in Blossom docs.
See MGNLUI-7100 and closing comment (as "workaround exists").

[...] We gratefully appreciate that the Merkle team has now gone the extra mile, by open-sourcing and publishing their UI 6 Definition Builders library on GitHub => https://github.com/merkle-open/magnolia-definition-builders, at the benefit of the broader Magnolia & Blossom community. Thank you guys!

We reckon that this will be the library of choice for Blossom implementations on Magnolia 6.2 moving forward.