Perhaps you are right in principle, but I think advocating for degrowth is entirely hopeless. 99% of people will simply not chose to decrease their energy usage if it lowers their quality of life even a bit (including things you might consider luxuries, not necessities). We also tend to have wars and any idea of degrowth goes out of the window the moment there is a foreign military threat with an ideology that is not limited by such ways of thinking.
The only realistic way forward is trying to make energy generation greener (renewables, nuclear, better efficiency), not fighting to decrease human consumption.
This being said, I think that the alternatives are wishful thinking. Better efficiency is often counterproductive, as reducing the energy cost of something by, say, half, can lead to its use being more than doubled. It only helps to increase the efficiency of things for which there is no latent demand, basically.
And renewables and nuclear are certainly nicer than coal, but every energy source can lead to massive problems if it is overexploited. For instance, unfettered production of fusion energy would eventually create enough waste heat to cause climate change directly. Overexploitation of renewables such as solar would also cause climate change by redirecting the energy that heats the planet. These may seem like ridiculous concerns, but you have to look at the pattern here. There is no upper bound whatsoever to the energy we would consume if it was free. If energy is cheap enough, we will overexploit, and ludicrous things will happen as a result.
Again, I actually agree with you that advocating for degrowth is hopeless. But I don't think alternative ways forward such as what you propose will actually work.
If humanity's energy consumption is so high that there is an actual threat of causing climate change purely with waste heat, I think our technological development would be so advanced that we will be essentially immortal post-humans and most of the solar system will be colonized. By that time any climate change on Earth would no longer be a threat to humanity, simply because we will not have all our eggs in one basket.
But why do you think that? Energy use is a matter of availability, not purely of technological advancement. For sure, technological advancement can unlock better ways to produce it, but if people in the 50s somehow had an infinite source of free energy at their disposal, we would have boiled off the oceans before we got the Internet.
So the question is, at which point would the aggregate production of enough energy to cause climate change through waste heat be economically feasible? I see no reason to think this would come after becoming "immortal post-humans." The current climate change crisis is just one example of a scale-induced threat that is happening prior to post-humanity. What makes it so special or unique? I suspect there's many others down the line, it's just very difficult to understand the ramifications of scaling technology before they unfold.
And that's the crux of the issue isn't it? It's extremely difficult to predict what will happen once you deploy a technology at scale. There are countless examples of unintended consequences. If we keep going forward at maximal speed every time we make something new, we'll keep running headfirst into these unintended consequences. That's basically a gambling addiction. Mostly it's going to be fine, but...
The only realistic way forward is trying to make energy generation greener (renewables, nuclear, better efficiency), not fighting to decrease human consumption.