Here I share a way that might be usefull. It is to use a dedicated keyboard with it's own clipboard. Copied text there is not synched to system clipboard and thus not accessible to other apps. Not all keyboards clipboards are isolated like that though.
I found florisboard has a clipboard that's not visible outside of florisboard. You can check that by copying some text by keyboard's copy shortcut and then try to paste somewhere by paste from the context menu (prompted by long press) you will paste not what you copied by fflorisboard, but something that was in you system clipboard. And if you need to paste what you copied with florisboard you can do that only by paste command from florisboard. That makes me think florisboard's clipboard is not accessible to other apps. Correct me if I'm wrong please.
Remember to set clipboard setting in florisboard to NOT to synch with system clipboard.
Heliboard's clipboard is available to other apps, so might be other keyboards.
I only tried these 2, there might be more with the desired by OP functionality. Also worth noting, above method is limited to only text from window where you can pull up the keyboard to be able to use it's clipboard. So some windows with text, but no field for text input, wont allow above method of copying. That is my ecperience, maybe there is a way to prompt the keyboard opened on any window that I am just not aware of. I hope it makes sense and helps, I tried to explain at my best.
I hope it makes sense, I tried to explain at my best.