How much time per day should I be spending with my cat?

My cat is 6 years old and a total sweetheart … when he wants to be. Sometimes, though, he gets extremely needy and obnoxious, demanding a lot of attention from me. I want to make sure he’s stimulated and happy as best as I can, but I also have to balance my professional and personal life too. Hence the question – how much of my day should I actively be spending with my cat? (I’m writing this during COVID, so I work from home at the moment, which means I spend almost all of my time at least passively with my little buddy.)

He is a well-trained primarily-indoor cat. Our current routine is this:

• In the mornings, before I leave for (or start) my daily activities, I put him in a harness, grab a lead, and take him outside. I don’t let him roam unsupervised if I can help it. (I live in a suburban area near a major state road, but also near relatively untamed woods.) I don’t keep him on the lead while he explores, but follow him around for about 20 minutes before bringing him back inside.
• Depending on the day (that is, if I have time) this might be followed by a small training session, where he is rewarded with treats for doing various tricks such as high fives. Usually only 2-4 tricks/treats at this time; takes a couple minutes. Sometimes I try to work in a few minutes of coat-brushing too (again, time permitting).
• After dinner, another walk.
• He usually settles in for an hour or two after this, but will start demanding attention again a couple hours before bedtime. I sometimes do another round of tricks/treats, maybe brush his coat, but pretty much always I pull out a toy to play with. (This gets mixed results as he gets bored of toys very quickly…) The whole process typically lasts 10-20 minutes.

This doesn’t include occasional mid-day work breaks to give him brushes and try to play with him. He can get quite demanding, and vocal about it.

All in all I estimate I spend a total of 20-30 minutes actively interacting with my cat in two sessions, plus another 40 minutes passively letting him patrol the area, getting into (harmless, supervised) trouble. Yet, this doesn’t seem to be enough for him, and he often expresses boredom vocally (first with higher, more demanding calls directly at me; but if those don’t meet with the desired response, they sometimes shift to sadder calls).

So, back to the question: how much of my active time should I spend with my cat per day, to make sure he’s getting the attention he needs? I’ve read online that I should spend 20-30 minutes per day of stimulation, but mine seems to want far more than that.