.NET CMS that handle complex surveys
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Our customer wants to migrate from **DotNetNuke** to another open source .NET CMS. I'm looking for something that will have the same advantages as **DNN+Dynamic Forms/Registrations** - ie. possibility **to create complex surveys with events** (showing one field based on response from another, auto-submit if specific answer occurs, events on survey submit like SQL query execution, HTTP POST or e-mail sending). 
Right now there are thousands **(around 50-100k) active users** (each user logs in and has her/his personal data stored in DNN surveys), so this CMS should be capable of creating rather big solutions. 

What are the options? Should we stick to DNN considering migration to a newer version? Are newer DNN versions worth the rework (we're currently running on 4.9.5). **Are there any open source and free .NET CMS-es that are comparable to DNN in terms of survey creation, and are better option when we think of usability, performance, stability, etc.** - basically all the fields where DNN more or less fails? We were thinking of trying out **Umbraco** - does it have a good survey module?

Any suggestions will be much appreciated. 