There is little to no downside to turning extra copies for units I have already +10'd into manuals, right?

In my case I intend to make every character 4*+10 if I'm not going to make them 5*+10 or they don't have fodder, this way if they get a good refinement I already have them ready to use, after I get them to 4*+10 I'll transform every copy of that character into manuals untill I get 10 copies as I may decide to 5*+10 them in the future, after that every extra copy will be feathers

If you turn a unit into manuals they lose merges (not sure if that’s what you’re asking). And what it does is it’s takes that character and turns them into an unusable but can be used for fodder or merges later. So if you don’t plan on using that exact copy of that character then you are good

If you're sure you won't use that Hero in the future then no.

But one thing about turning them into manuals is that they lose their IVs. So if someday you want to change the IV of one of your +10 hero and you have turned that hero's extra copy with the newly desired IV into combat manuals, then you can't until you get another one.

Example:
Current +10 IV: Neutral. Then you want to change to +Atk/-HP. You had an extra copy of that hero with that IV, but you had changed that copy to combat manuals. So now you can't change the neutral IV until you get another copy of that hero with +Atk/-HP.

So my advise is, if you want to change to combat manuals, better check their IV first in case you need them in the future.

Hope this helps :)