I don't speak for Nintex, so someone may refute this but I do not think there are any plans to add support for Visual Studio 2017 or above. The original plan was to obsolete the Visual Studio designer and have customers migrate to the Web Designer. There unfortunately isn't a tool to do this migration automatically, so it will take some development effort but workflow pieces (such as calls to SmartObjects) can be reused so it will not be as much effort as redeveloping from scratch. I have performed a few such migrations without issue, the main feature available in the Visual Studio designer that does not exist in the web designer is custom code - such logic would have to be moved to a web service or other integration point that can be invoked via SmartObject.

We are in the exact same position. We have built up a bit of architecture around K2. We used Server Code Events exclusively to call back end services based on performance issues we experienced in 2009/2010. The recommendation by K2 was not to use SmartObjects but use the code event.

Anyway, we are in the process of migrating all 500ish workflows from K2 Blackpearl 4.6.6 to K2 Five. We have leveraged a conversion tool and modified it to replace code events with Smart Object calls. I'd be happy to connect and share our story and what we are doing to get past this problem.

There are integration points in K2 4.7 that's not available in 5.5.

There is no SmartObject to call on an Email event.

There is no Process Finish rule to ensure the process does not terminate unexpectedly.

There is no passing of K2 context, which allows custom code to gain access to process properties.