I had the same issue and after searching for a long time I found a solution to delete specific pictures or videos from my phone's memory (but keep them showing in Samsung Gallery and in OneDrive, with sync activated) and not using "Free up phone space" that removes all files from your phone's memory.
I don't remember where I found the instruction but what you can do is use the "My Files" app that you have on your Samsung phone and then find the files you want to remove from your phone's memory but keep showing up in Samsung Gallery and keep in OneDrive. You can delete the files there and it will tell you (at least it did some time ago) that it will delete the files also from other apps like "Samsung Gallery" but it's not true. If I remove them from My Files, even after deleting them also from "Trash" it still shows those files in "Samsung Gallery" and the files don't get removed from OneDrive, they are just shown with a cloud symbol in "Samsung Gallery".
I was scared in the beginning, as I expected that the files might disappear after I delete them in the "My Files" app but I tried first with few files that I did backup before trying it and then synced again after deleting them to be sure. The files were still in OneDrive, they didn't use any memory of the phone anymore and they still showed up in the Samsung Gallery app. For videos it's unfortunately very slow or not working nicely if you try to watch videos that are not in the phone memory. It will show information "Play cloud video" in Samsung gallery app. The files still stay synced just not downloaded to the phone.
As the videos are usually what takes up most of the space, I usually use the "Manage Storage" functionality in the "My Files" app and then go to "Large files" and set the threshold to 100MB, 200MB or something like this. Then I see all the big files, like mentioned those are usually the videos, and can remove them from there. And then delete them from Trash, if you have trash activated. Like this, I can release a lot of memory quickly but still have them in the Samsung Gallery app. As mentioned before, watching videos from the cloud in "Samsung Gallery" app is veeeeery slow and even when it starts, it gets stuck a lot. If I really want to watch those cloud videos I usually use the OneDrive app and watch it there, as there it works pretty well. If I want to watch a specific video, i can also check the file name in "Samsung Gallery" app and then search for that file in OneDrive, if it's hard to find in big folders.
I would appreciate if Samsung or Microsoft (OneDrive) would improve this "Play cloud video" functionality to make it work properly and not get stuck every few seconds, as it's useless for me as it is. Until this happens I need to use the OneDrive app to watch those videos that I don't have in the phone's memory.
