M5 Ultra if the portable form factor is not a requirement. (I have a M5 Max 128GB 2TB for context.)

That being said: you might want to consider at least 2TB storage. Having 256GB RAM to fit models is less useful when you can only store a 3 or 4 large models on your disk. The M4 -> M5 transition doubled the drive bandwidth (at least for the MacBook Pro, I'd assume the Studio is no different) making them extra nice for loading large models. Then again, you can always add an external NVMe drive over thunderbolt, so the tradeoff is less straightforward than with a portable MBP.

Contact Apple care and see what they can do. They have been known to do upgrades if you buy around the same time. But with the ram situation that courtesy might be gone now.

You should be able to cancel it for free.

In EU you also have 14 days to return products bought online, for any reason.

M5 Ultra and toss in a MacBook Neo or older MacBook Air to SSH in