Interestingly your last sentence is exactly what I just tested and wanted to “report” here - If I import only the mesh of the UE4 standard mannequin in blender, apply “my” armature to him and get both into the engine, using the same skeleton asset I created for my original character it works just fine to also use the same animation for him as I used for my much smaller and wider character. So it will probably work to once retarget all animations to “my” skeleton and then reuse those assets and a single Anim blueprint for all characters I create (who are similar enough so I can reuse the skeleton) which is exactly what I wanted in the beginning (except for the first retargetting step but the engine is so simple here that it doesn’t hurt in any way).
Once again, thank you very much, especially for the further info which helped me understand the problem a lot better.