If anyone is planning to upgrade from K2 4.6.11 to K2 Five, this is a summary of the approach we took and some of the issues we faced. https://ozippy.wordpress.com/2019/01/29/upgrading-k2-4-6-11-to-k2-5-2/
If it's any consulation, i'm finding the 4.7 to 5.2 upgrade equally as annoying. Post upgrade, there are a tonne of performance issues and bugs.
The biggest thing i agree with you in the complete lack of thought gone into producing a thorough and thoughtful upgrade path and documentation. Yes, everyone uses different web services, form methods, scripting etc, but a lot more thought should be put into K2 producing an upgrader that caters for their previous products in a way that everything is supported.
Anyway, i continue to find bugs and weird things on my Dev box that do not work and will post a full write for others on here. I don't have the luxury of a team to rely on, this is all trial by testing for me.
Essentially, every major K2 upgrade ends up as 6 months of fixing bugs and testing all previous forms and workflows, than doing any new design work or new forms. K2 should take note of that !!!