I think we can go higher with stuff like deserts covered in glass algae growth tubes. And things like fusion fueled vertical farming are still eventually on the table.
If you covered all land with nuclear powered greenhouses we could have enough food for a trillion people. Not that we should, but it's a thought exercise showing that food isn't the limiting factor for population. I'm really glad that we've found another limiter, because imagining life on an Earth with a trillion people.
I hope the solar system is someday home to a trillion people. We need more minds working on the problems the universe presents to us. If humans won’t step up to that challenge then maybe our silicon offspring will.
I used to have similar hopes, but I think our first priority must be increasing efficiency of research. Consider the transition to open access; it is still a minority of research effort so many researchers waste untold resources reinventing the wheel. Also the incentives to publish mean a lot of fields consist of people writing papers for the sake of writing papers, without much progress towards anything resembling novel ideas