Image attachments

Hello,
I need help with image attachments.
I’m creating a single post page with the proper (post) widgets.
Thge goal is, that the user (not experienced author) should be able to post image (featured image of the post), galleries and videos easily, without having to learn WP, if you know what I mean. So far I installed ClassicEditor over GuttenbergBlocks (which is absolutely unusable even for me BTW), featured image is also “solved”, should be no problem for this unexperienced author to upload the image. The problem is however with galleries. I did found that the gallery widget has the ability to show attached images as a dynamic content, and it works actually, but I still didn’t found easy working solution, which will allow the regular user simply pick a couple of images in the post editor to attach them. Tried WP Attachments plugin, but it doesn’t look like working so far (in terms of usability for an unexperienced user). It doesn’t actually allow (show) user to pick images from media gallery but forces to upload – could be very confusing. Is there any simple, working way to attach images to a post with couple of clicks? So the result should be like this: the single post template will have gallery widget in it. If the post has attached images – there is gallery, if the post has only featured image – gallery is empty. Thank you!