Do physical and digital copies read the same data?

Both things use the same data, as all your save files are in the cloud... everything is always backed up. So, if you ever switch you'll never have to worry about losing data. The only real difference, I guess, is convenience. Both digital and disc act
like you own the same licenses when playing said games. I personally prefer discs, they look nice and feel more reliable than digital.

It should read the same data if you haven't deleted any saves.

Yes, doesn't matter if it's buying the physical and then the digital, or vise versa, it's fine. I only know from my recent experience with that similar situation.

Great! Last but not least, if I had a digital game that was delisted and it was uninstalled itself but just in case I also play the physical disc copy of the same game, I won't lose all of my progress, achievements and unlockables and can go on with the
disc instead right?

With physical media like discs they take time to read the disc and hdd, as digital media only reads the hdd.