I think that every day they are clean is a miracle and 90 days is a marker for them, so perhaps a nice card would be appropriate.
I don't think I would buy a gift until one year, but if they lived near me I might take them out for dinner or just some special "treat".
And I understand your caution about getting too close. We codies have been riding the emotional roller coaster too long and find that we don't have to let ourselves be vulnerable, not even emotionally, until the time is right for us.
Hugs