but the existing drywall lines from when the drywall was installed, in 1969, are still showing

I don't see any drywall lines in your images.

What are you trying to show in the image with the two red lines?

Are we looking at painted ceilings?

I see what looks like some imperfections. Is this what you are asking about?

Sorry my pictures may have been reduced in resolution after uploading.   The red lines show the dry wall lines and your circles show the areas they may have smoothed over or old nail smoothings.

To answer your question,no amount of paint can hide poor mudding and sanding job.

Looks kind of like my bathroom ceiling which i patched myself lol. our contractor took completely uneven old plaster paired with drywall of totally different thickness and made it look flat. same for drywall team that did our kitchen (again matching plaster and drywall flawlessly). it doesn’t look terrible in the pics but painting isn’t going tomake it go away. i’d definitely choose flat paint!

Impossible to know what is going on from just pictures.

Could be that bottom barrel cheap primer was used and topped with one coat of bottom barrel cheap ceiling paint and the new mud work sucked up the cheap paint. Or possibly being stingy the the paint application.

Or bad drywall work & any or none of the above.

Use some kind of straight edge--large drywall knife, metal yardstick, etc. and place an edge perpendicular to the ceiling across the questionable areas. That will tell you how flat the ceiling is & what the problem is.

Add thin drywall to the ceiling