LimeSurvey

 LimeSurvey  is an open source survey tool also available as a service with a decent UI, that can be self-hosted on an IndieWeb site.

How to self host
See https://manual.limesurvey.org/Development_overview

tonz
has been using Limesurvey self-hosted: It works well, but has a convoluted interface to create questionnaires. In a previous company we worked well with the Limesurvey team in Germany to create specific new question types.