This idea makes a lot of sense, after you've decoupled health from age to a much greater degree than today. However, at that point I think the better idea would be "There is no longer a compelling reason for retirement or long gaps between jobs to be a matter of public expense at all, unless you're doing it for some set of purposes the public cares about." I think we need to normalize the idea of years-long gaps on resumes, among other things. But I don't feel compelled to share the expense of different people's career paths and life choices.
The reason we originally set the retirement age approximately equal to the average lifespan was to ease the burden on people planning for their own futures, so that no one had to suffer for living longer than they'd expected. The fact that we now live many years in relatively poor health near the end of our lives is a very different situation. and we've adapted the policy tool we had to try to cover it. The idea of retirement as a long vacation or reward for having worked 40 years is an anomaly. In the near term, we shouldn't force people to work when they're not able to, regardless of age, but we also maybe shouldn't be paying living costs just because someone hits a certain age. Protests and strikes aside, I think a better solution would be to eliminate age-based pensions, and greatly expand disability-related and poverty-related social security programs to the point that everyone who outlives their ability to support themselves is still covered. If you like, phase it in gradually over the next 30 years so that anyone not near the beginning of their careers doesn't have to worry about it. I realize this would still be a political firestorm, of course, but as noted, this is a scenario where every available option is considered unacceptable to large swaths of the population.
Strictly speaking, Kardashev level 1 requires control over a whole planet's energy budget such that we are capable of using it. It says nothing about what we do with it. "Choose to not use it and leave some spaces wild, when we could easily choose otherwise" seems like a perfectly valid way to meet that criterion (that we don't yet meet).
Right now land costs are on the order of $1k-$2k/acre/yr (1 acre ~ 4000m2, but I find it a convenient metric because an average acre receives an average of just over 4MW of sunlight if you spread it across the full 8760 hours in a year, which gives an average of ~1MW output at current efficiencies if you had 100% panel coverage). and with current efficiencies in typical regions that's something like 2000-8000 MWh/yr depending on local weather and panel layout, so <$1/MWh. If we move towards tandem or other multijunction cells (which seems plausible in the 2030s) that power density could double. In addition there are some slower trends that should start to support things like agrivoltaics (dual use of land without decreasing crop yields) and comparably cheap or cheaper non-silicon semitransparent panels (which can actually be used in greenhouses or over crops, selectively absorbing wavelengths plants can't use while providing shade to reduce water consumption).
In other words, there are lots of options to address this. World electricity consumption would have to increase by at least 3 orders of magnitude before land use even started to become a consideration.
I do think the OP is overestimating the rate at which energy storage and synthetic fuel costs will fall, and that that is a bigger consideration than land use. I also think resistance to early retirement of existing assets will slow down the later stages of the move away from fossil fuels, both in electricity generation and in transportation fuels. But I doubt that shifts the overall timeline by more than 5-10 yrs.