We would like to use an interactive system for polls in our lectures. What would be alternatives to ahaslides.com that are e.g. opensource or self-hostable?
According to alternative.to, there are multiple open source competitors apart from PINGO:
PINGO might suit your needs (see also the list of features). The project is (or was) associated with the German Universität Paderborn.
The source code is available and licensed under the Eclipse Public License v1.0 (which is an OSI-approved open source license).
A (German) blog post by the PINGO developers mentions that using the source code, you can technically host an instance yourself, however, they don't offer any support.
I don't know about open source, but as I understand it, Kahoot!, Google Forms, Poll Everywhere, Slido, and Mentimeter are all integrable into Google Slides, and all have free tiers that allow you to do what you are asking for.
The advantage of the freemium services is that they are simple and tend to work out of the box.