It's a good question and the answer is
maybe. If your SDV+ Arcade save file is stored in the Files app > On My iPhone (or On My iPad) > Stardew Valley, it might work. Here's some thoughts and hopefully I'm not treading on stuff you already know. Sorry it's a bit long.
- I believe the Arcade version uses Game Center to auto-save game data to iCloud. This allows you to install the Arcade version on multiple devices (iPhone/iPad, and for some games even Mac/AppleTV) and have the same Arcade version game data available.
- The Arcade version is 1.5.6.46, and the regular iOS version is 1.5.6.39. Not sure if that creates a problem.
- Unlike Game Center, the Files app gives you access to actual files/folders on both iCloud Drive and On My iPhone (or On My iPad) under Locations. You can move files/folders from one location to the other easily within the Files app.
- I recommend copying your save file folder (farm name_number) from your device to iCloud temporarily as a backup. Not a bad habit to do regularly anyway. This is different than Game Center saving your game data automatically to iCloud in a location to which you don't have access.
- It won't allow you to copy the whole SDV folder, so open the folder and copy the specific save file folder to iCloud.
- Once you've selected the save file folder, the option may show as Move but it will, in fact, just copy it to iCloud.
- Uninstalling/deleting SDV from your device removes the SDV folder. Install/reinstall the game, and open it without creating a new save to create the SDV folder in the On My .... location.
- BUT uninstalling the Arcade version won't delete the game data saved to iCloud via Game Center. It will still be available if you reinstall the Arcade version. Some non-Arcade games also use Game Center, but the regular iOS version of SDV does not, only the Arcade version.
- You can then copy down your save file folder from iCloud back into that On My ... SDV folder, and it should show when you tap on Load after launching the game.
Hope this helps. I'll be curious what you find out. At least the regular SDV version is a nice price.