Hi Tobias,
I tried to autoplay the embed video in iframe in reactive app. I was able to achieve the same. Below is the screenshot of options i gave in iframe tag.

What I found is, if we are using autoplay=1 only in src it's not playing the video but if we are using autoplay=1 and also mute=1 together. autoplay is working.
Hope you will achieve this too. Please refer the screenshot attached.
visit this url for live example
https://msingh.outsystemscloud.com/Test/