Thanks. I would love to stick with srcset in this particular case. However I understand this might not be possible, just found the essential information »From a previous question I understand the srcset does not upscale images« from this thread Responsive images questions - #3 by Mark_E
I fixed the resulting layout-problem with css :
aspect-ratio: 4 / 3;
object-fit: cover;
object-position: 50%;