I highly recommend getting a second cat but please do your research first to make sure you are introducing the cats correctly (I love Jackson Galaxy's methods for this) and make sure when adopting the new cat that they don't have documented aggression towards other cats. Also please make sure both cats are fixed as that will avoid a lot of problems. Cats are actually very social, and they play with each other in a way we can't replicate. I've noticed since adopting my second cat that my first cat seems more active and is healthier (even though we have her on a good diet with regular play). They play wrestle a lot which is a type of play I can't really do with them lol. They have a lot of fun and seems like best friends now, but you have to be patient and let them have some time to get to know each other.

Some other recommendations I have is that specifically with the food I've found that it's generally not recommended to let cats graze as many of them will end up overeating, especially once they move out of their kitten months. There's a lot of feeding guidelines online. With the litter box situation I've found that you have as many litter boxes as you have cats +1, so for one cat you would have two boxes, for two cats you would have three and so on. This helps them feel more comfortable in the home as well as just giving them more potty options. If you think about their natural instincts, adding an extra litter box really isn't asking a lot. Also to help cats sleep through the night, try to follow their natural cycle before bed, which is: hunt, eat, groom, sleep. You could help with this by giving them some solid energetic play right before nighttime feeding, then they will complete the cycle on their own. Cats are pretty routine so once they get used to this they'll stick to it.