Finally got a proper answer from Rockstar (lesson learned; the support chat is useless…)!
Thought I could share it here, if someone at some point is wondering the same thing:
”At this time, the file size for RDR2, including Online only updates is roughly 150 GB. As such, after installing the game from the disc, your PS4 may need to download an additional 50-60 GB of data in order to play the game. This may vary, as some files are replaced and removed with updates and patches. As such, we do not have the exact size of the patches and updates after the initial launch of the game”