Yes the belts could be the issue. Idealy you want belts that are not out to the extreme of adjustment when new as they typically stretch during use. So you would have no further adjustment if they were even tensioned properly at the extreme of adjustment and yours are not.

Anyways the fault or not they have to be dealt with. You for certain should not be able to turn the alternator pulley with fingers on the fan portion of the alternator.

The 40mv drop was low enough to establish the ground to the alternator is good enough. So you are reduced pretty much to the loose belts that I suspect may be the issue. Or the alternator is no good as well basically.

An alternator asked to give a high charge initially especially draws a lot of mechanical power through the belt drive. The belts have to be set up to transfer that much energy. You did put new belts on I think. So take the old ones and read their original length in the code on the flat edges may give some help if still visable and it usually it is.