I give and have given my buns local hay for a very long time. I only give pet store hay when I cant get local farm hay. It’s horse hay, a mix of grasses, mainly timothy but not all timothy. The only observandum with horse hay is that it sometimes contains quite a bit of alfalfa. Alfalfa is good for horses but too rich in protein and calcium for adult rabbits.
A rabbit that gets a lot of human attention can live happily as an only-rabbit. Rabbits as a rule benefit from a same-species friend, but not all rabbits will accept another rabbit.
It’s good that you’re keeping an eye on your buns weight! Weight is a very important indicator of a buns health, since they’re very good at hiding any type of illness or discomfort. Unexplained weightloss in a young-ish bun can be caused by many different things, ranging from easily fixed ones (tiny molar sprs f ex) to more serious ones (f ex kidney or urinary tract problems).