There are two parts to this: (1) just being able to read the floppies; (2) making sense of the files on them.
Actually, I think the best solutions to your problem, Carl, are ones that involve the use of an Atari ST (if you can find someone with one).
Re ‘(1)’, the later versions of the Atari-ST OS DID actually format floppies in a way that they could be read by PCs. I take it that your Atari ST didn’t do that. There were Atari utilities that would modify a floppy to make it readably by PCs - though I’d be worried about using those with the only version of a floppy. I presume you don’t still have an Atari ST? If you do (or if you can find someone with one) that might be the easiest way to get them readable on PCs and/or to save files that C6 can open.
Re ‘(2)’. That’s the harder part. I don’t think you’ll be able to run Pro-24 in an Atari-ST emulator on a PC (like you were hoping), because of the missing dongle. Lean a bit closer while I whisper something - perhaps a cracked version would run? However, you might not need to use Pro-24, because some versions of Cubase used to open Pro-24 files - I can’t remember when it stopped - whether it was just Atari versions or if Cubase VST would open them. If it’s the latter, perhaps you could find someone who still has access to Cubase VST and would be willing to open the files and save them in a suitable format?
A warning about the floppy disk formats …
The standard Atari ST disks were only single-sided and not high-density. Some modern drives don’t claim to read such disks, while others do claim to. However, I’ve been trying to buy an external USB floppy drive that will read 720KB disks, but, having had three of the same model from Amazon UK only to discover that they won’t read them - in spite of an assurance that it was just a problem with the first batch - I’m wondering if I’ll actually be able to find a suitable drive. (I’ve been able to borrow one that works - so they were available at one time.) My older PCs that came with a floppy drive do all read the 720KB format.
I sometimes used to use utilities on my Atari ST to format disks with greater capacity (eg more sectors per track and/or more tracks). I haven’t tried reading those on a PC, but perhaps it’s impossible? So if you did that too, you might have an extra problem to deal with.