Your last choice should not work, or at least didn't the last time I tried it, because Windows has "gotten smart" and if a second screen is not detected for actual use, it reverts to the primary screen.
As to the other two, it really depends on how the screen in a given machine handles things. For some reducing brightness does not really change anything in regard to the backlighting, which is what eats up a lot of power. I think the same is true of screen curtain functions, at least to an extent. They have to be dependent on how hardware handles things.
It is definitely possible, though I cannot say that it's a good idea, to disable your monitor/display from within Device Manager. I won't even try it because I don't want to take the slightest chance that I'd end up in a position where I have no display nor the requisite skill to turn it back on very quickly using the screen reader once it's off. Someone else here should be able to comment about whether this approach works and whether it's advisable or not. Were you to do it, and were the result persistent between boot cycles, you would have to go back in to Device Manager to re-enable the monitor if it were to be needed for sighted assistance.
Brian - Windows 10 Pro, 64-Bit, Version 20H2, Build 19042
The depths of denial one can be pushed to by outside forces of disapproval can make you not even recognize yourself to yourself.
~ Brian Vogel