It sounds like you have done a good job so far of providing basic opportunities for engagement. I have two tips that you might not have thought of yet.
If you live in an apartment complex or neighborhood, there might be older children around who would love to hang out with your cat for free or very cheap. 10-15-year-olds are ideal for this, especially if you know their parents. College students are another great option. You might be able to enlist a young neighbor to come over while you are doing some work, making dinner, etc. or even before you get home once you trust them. They can spend half an hour playing with and generally spending time with your cat.
Bringing in guests has a few benefits. Obviously, this solves the issue of not having enough time to play every day. But additionally, a new person brings new smells and new ways to socialize. Their presence is stimulating in and of itself.
People don't often think of training as a fun activity for the animal, especially for a cat. But really, training your cat to do tricks or behaviors can be a fun game for everyone. From the cat's perspective, it's a puzzle where they get to figure out how to get you to give them the thing they want. From your perspective, you get to teach your cat to do cool, funny, or weird things.
The key to persuading a cat to play this game is to figure out what motivates them. For most animals, food is a good motivator, although you have to find something that they really want and that they don't get the rest of the time. There are lots of cat treats for sale in pet stores, but some cats aren't enthusiastic enough about the dried ones. Bits of fresh meat often work, or tiny pieces of hotdog or fish. Think about your cat's favorite foods and figure out which inspire your cat to come running. That's what you want to use.
With the right motivator, you can train most cats to do almost anything a dog could learn and then some, although many cats will need to be in the right mood to follow directions. But 15-minutes a day of playing the "how can you get me to give you this tasty snack" game adds a lot of mental variation to your cat's life.