STM32L073xx Internal Bootloader - Get Flash Size issue
It's unclear why reading address 0x1FF8007C does not work, I get the same failure.

However, experimenting with a Nucleo-L073RZ shows that it will permit one to read 128 bytes starting at address 0x1FF80000, which will have the flash size value which you seek as its penultimate 16-bit word.

Thinking it might be an offset issue I tried reads at 0x1FF80000 + [0x70, 0x60, 0x40] none of which worked.  Conversely, reading at 0x800007c does work, so offsets are allowed in general.  And as the read size is sent after the point where the address is NAK'd, it's not an issue with a 32- or 16-bit read being required.

Anyway, reading out a 128 byte block should solve your problem.

Another idea would be to start at an address near the top of the largest flash size and read down until you stop getting a failure.  Or you could load a little stub into RAM and execute it.  But it seems like reading out 128 bytes will be your simplest solution.

As an aside, using https://github.com/florisla/stm32loader (which seems to be what is on pypi, you didn't say what you were using) I found that I had to reset the chip each time I invoked the program, so apparently it is not leaving the bootloader in the right state. Still, that is an improvement over another tool I'd tried a few months back, which refused to talk to the L07x bootloader at all, though it was fine with the L05x one...

If one really wanted to spend an afternoon descending a rabbit hole, it would probably be possible to set a breakpoint before sending the address CRC byte and trace the bootloader operation to the logic which generates the ACK or NAK, but as it's in ROM all you would probably learn is precisely what is or (apparently erroneously) is not allowed.