I raised several foals at an urban boarding barn who were kept alone with just a horse on each side. They got ample hay but not 24/7. They got turned out into the arena but we had no pastures. they had a 12x12 stall with an adjacent small paddock.
They were not only happy, well adjusted and healthy, they were also a joy to work with and had no issues from growing up that way. Did I wish I had acres of grass and several other babies for them to play with? Of course, but sadly that isn’t always the case.
OP is doing nothing but right by their 18 month old horse and putting a 6.5( BCS horse) on unlimited hay is more abusive then not having a playmate.
We have to work with what we have available to us. There are many ways to raise a young horse right.