Target display extremely dark

Except that I just got it to work (at least once) by rebooting the laptop. That could be coincidence, but it hasn't worked in several days, and after I rebooted the laptop it worked the next time I tried. I'll find out shortly whether it still works the second and third times after rebooting....

Some more experimentation, and I've learned several consistent things:

• The first time I hit command-F2 on the iMac after rebooting or logging in on the laptop, life is good.
  • When I open Display Preferences, a window appears in the middle of each display, entitled "Built-in Display" on the laptop and "iMac" on the iMac. Each such window has a brightness slider (among other things), which works as expected to control the brightness on the appropriate window. Even if I drag or "gather" the iMac display window onto the laptop display, it still controls the iMac's brightness, as I would expect.
• The second and subsequent times I hit command-F2 on the iMac after rebooting or logging in on the laptop, life is bad.
  • When I open Display Preferences, a window appears in the middle of each display. At least, the iMac display has a faint rectangle the right size and shape that wasn't there a moment ago. I can't tell whether it has a brightness slider because it's too dark. I can't drag it to the laptop display because it's too dark to see the mouse. But I can "gather" it to the laptop display. And it has no brightness slider at all.

(I'm guessing that the lack of a brightness slider and the extremely-low brightness setting are two effects of the same cause.) Anyway, I think we now have a system software bug on the non-target system: after the first time per login session that a target display is connected and disconnected, the display preferences for that target display no longer have a brightness slider, nor any brightness at all.

BTW, both computers are running 10.15.5.

Moved that file to my homedir (on the iMac), rebooted (the iMac), and there's now a new (presumably factory-standard) file in that location. But the symptoms haven't changed: I go into target display mode, both displays go black for a second, then the laptop display comes up and the iMac display doesn't. I can drag a window onto the iMac display, and if I look closely I can tell that something with vertical and horizontal edges is moving (it's almost as bright as the fingerprints on the screen surface).

I doubt it's the ambient light sensor setting, since both the iMac and the laptop look just fine when one isn't acting as a display for the other. I'm typing this on the iMac now (not in target mode), and it's as bright as one could wish. I thought at one point that positioning the laptop in front of the lower-right corner of the iMac might be the problem, but now that I have a longer Thunderbolt cable I don't have to do that, and the symptoms are unchanged.

I tried plugging in my wife's laptop instead of mine, and got the same result: both screens go black for a second, the laptop screen comes back up, and the iMac screen doesn't. So it's probably not a hardware or software problem with my laptop. I've tried two different Thunderbolt cables, so I don't think it's a problem with the cable. I've tried both Thunderbolt jacks on the back of the iMac, so I don't think it's a problem with the jack.

Yes, I'm aware of the article "Use your iMac as a display"; that's where I started this journey, and as mentioned above, I've gotten it to work a few times.

As mentioned above, the iMac display's brightness, resolution, and color profile are just fine when it's not acting as a target display, and they were just fine the few times I got this to work. But MOST of the time (yes, at least 90%), it's almost entirely black. I've tried adjusting brightness in the "Displays" settings pane on the laptop, but it only affects the laptop display, not the iMac target display.

First question from BDAqua: I don't see a way to choose/highlight different displays. I go to Settings -> Displays on the laptop, then see a window entitled "Built-in Display", which has three or four tabs: Display, Color, Night Shift, and possibly Arrangement depending on whether or not the iMac is currently in target display mode. If so, then there's an Arrangement tab, which shows the 27" monitor and the 13" laptop monitor, and I can move them around relative to one another, and the relative positioning works as expected: if I drag a window off the edge of the laptop screen, it shows up in the expected place (but extremely dark) on the iMac screen. Clicking on one or the other of the displays in the Arrangement tab causes it to highlight briefly with a red border, but has no other effect: in particular, it doesn't change the Display tab so I can (for example) change the brightness of a different display.

I tried checking the "MIrror Displays" checkbox in the Arrangement tab, and everything on the laptop screen became tiny, as though it had the pixel count of the 27" monitor, but nothing changed on the actual 27" monitor. Unchecked that again so I could read at least one of the displays.

Second question: I created another admin account on the laptop (or did you mean try another account on the iMac?), logged into it, tried all the same things, and saw no different behavior.

Just for grins, I also created another admin account on the iMac, logged into it, tried all the same things, and saw no different behavior.

Also tried all of this again with a different Thunderbolt cable. No different behavior.

Also tried all of this again with the Thunderbolt cable plugged into a different Thunderbolt jack on the iMac (which has two).

No, I can't. Clicking one of them highlights it (by outlining in red) for the duration of the click, but it doesn't stay "selected" in any obvious sense, and it has no effect on the "Display" tab, which is still about the built-in display.

If, hypothetically, I were able to "highlight" one of the displays in the Arrangement tab, what would I do to change its brightness?

On the iMac or the laptop?