Would my direct knowledge of SharePoint Designer and InfoPath Workflows help me to take on a third party solution substantially faster?
Of course, yes, Working with the available SharePoint tools like SharePoint Designer and InfoPath ... etc will help. At least, it provides you a very good knowledge of workflow and forms ...etc concepts,
At the same time that doesn't mean you won't need training on the new tools!
But the time it takes to learn and use the new 3rd party tools will be much less depends on your current SharePoint knowledge in general, the gained skills from using the available SharePoint tools in particular!
And I totally agree that the 3rd party tools are very helpful and make the SharePoint easier and faster compared to the available SharePoint tools like SharePoint Designer ....etc, but what prevents us to use these 3rd party tools is the cost factor!
What is the learning curve for such a solution?
This will depend on which tool you will use. but in general, most of these tools has its own official site, user guide, tutorials, and learning videos as well as a discussion forum like StackExchange to help you to report your problems.
Note: You should be aware of the technical questions on problems encountered using commercial 3rd-party software or plugins are
off-topic and will be closed in SharePoint.StackExchange.com as
they can be more easily answered by the specific vendor's support