The thing is, as much as we would want to have a specific thumbnail maker for the YouTube shorts, YouTube itself doesn't allow its users to have a thumbnail on Shorts videos. As we work on YouTube API, we need to make sure that we follow certain rules when it comes to development but even, If we do create the thumbnail maker for the shorts video, it won't add any good to the video since people can't see the thumbnail on shorts video.
When the shorts were first or newly introduced, having a customised thumbnail for the Shorts was a thing and YouTube did allow users and moreover, was letting people see the thumbnails but after recent updates, it's been almost 6-7 months since they have stopped supporting Thumbnails for the shorts. You can, however, still make shorts thumbnail on Canva or other graphic designing software but I'd rather wait and see If such new updates will be tweaked by YouTube at a later period or not, If YouTube, does decide to change or push updates when it comes to YouTube Shorts Thumbnail then this feature indeed could be useful, however, before that, it'd make no sense.
Anyhow, thank you so much for your suggestion nonetheless