If the issue with cross platform is that the save files on consoles cannot be accessed in the first place, how will you turn them into QR codes? Or are you trying to continue a PC save on console? I'm confused.
So someone was talking about wanting cross-save capability in SDV, and while I would like it if you could play the same save across multiple platforms, I doubt the various corporate entities would let that happen without demanding some sort of concessions that would effectively ruin it.
So I had the rather brilliant idea of encoding your save into a QR code that you could just scan with any device with a camera, and apparently every modern console has options for a camera (possibly overpriced, but still options.) The biggest issue I can see with this idea is that I have no clue on the difficulty of making some save-to-QR-and-back translator.
What I do know about QR codes is that they are effectively an odd method of writing text. When you scan one with your phone you can see that text, which often is a website address. SDV saves are just large-ish text files (I've done some minor save editing) so one could become a QR code. However I do not know how physically (pixelly?) large a SDV file would be if you just turned it one-to-one into a QR code. Eventually you'd run into an issue where there are too many pixels in your QR code for your camera to read, or your screen to display. (I don't think there are actual limits on how big a QR code can be, someone made Doom in a QR code.)
So two thoughts I had on making the QR more manageable is to remove unneeded text, and split it into multiple separate codes. The first is rather easy. SDV saves are readable, for example the type of farm you are playing on is saved as "0", with a different number depending on which farm type you are on (that is what I changed on my save editing.) You could just change all the various fields/names/titles into a number, or even a unique letter/number combo and drop down to 2-3 digits. This would make save files more corruptible, so I wouldn't have all saves be done this way, just the ones you are transferring.
As for the splitting it apart, I see no reason you couldn't have each QR code effectively start with "part X of Y of [farm name]" Have the decoder be able to read that and demand that you feed the QR codes in the right order (so if you give it part 2 first it complains that that isn't part 1, and then if you try to give it part 3 after part 1 it complains that it isn't part 2, etc.) Or have it be able to sort them into the right order, I dunno. If you really wanted to be fancy you could put an identifier code instead of the farm name, just to prevent people from mixing save files together, either accidentally or on purpose. Or at least make it harder to mix, as you likely could edit the codes somehow.
If this can work in the way I'm imagining it can, you'd edit up with around 6 QR codes per save that you could scan with any camera, and an in-game decoder that would turn them back into a save. I would hope that the PC version would be able to accept text and possibly run completely separate from the game, as then you'd be able to scan the QR codes with your phone and just email the text strings to yourself.
The other large issue is I dunno if the various consoles actually let their games read camera info. There was that xbox kinetic thingy back then, but I dunno if modern consoles actually allow games to see what the cameras see, or if it's just something that the console sees and walls it away from the games.
I am unaware of any legal reason why corporations could block something like this, as games should only be able to access their own save files, and not mess with anything else, which means that it should be ineffective as a malware vector. You would be able to play on saves that you didn't start yourself, but the only thing that would get you is achievements/trophies/whatever, as SDV is primarily a single player game, and who really cares about someone's gamer score or whatever?
I am aware that this could mess up in various ways, and I wouldn't recommend you do it often. But having the option to switch which system a save is on is nice.
The actual question bit:
If SDV files were about 1/2 their size (or even full size), how many QR codes would it take to hold all the data in a machine-readable quality on a smartphone sized screen? Also, how hard would it be for the console versions to be able to read one or multiple QR codes via camera and turn it into a save? Is there something like a QR code that would work better?
TL;DR: turn saves into 1 to X QR codes for the purpose of transferring them between platforms.
If the issue with cross platform is that the save files on consoles cannot be accessed in the first place, how will you turn them into QR codes? Or are you trying to continue a PC save on console? I'm confused.
QR codes aren't infinitely scalable, there's actually very little raw data you can manage to squeeze into a QR code. Making things scale down more efficiently is good, however Stardew saves a lot more than just a character sheet. If you have four chests, each with a dozen items, the save file will need to record every chest, every item, and quantities for those items, and where those chests are in the world. Now also consider every crop, every tree, weed, stone, wood, etc etc, *all* need to be recorded in the save file. Their coordinates, their status, their age. Every event flagged you've tripped, choices you've made, quest completed, etc. You could definitely save your own character as a QR code, maybe even your inventory too, but nothing beyond that could fit