Oh, you mean the Portenta product. Well, you left that part out, you only referred to the "H7" which is an ST product not an Arduino product. Good sleuthing work by @hzrnbgy did lead to an understanding, but it went over my head.
Have you tried just contacting Arduino LLC and asking about a custom order? With chip placement, the machines have to be set up, but if the packages are the same, it's just a change of reel on the placement machine. If you think about the ramifications of that, changing the reel and then running boards through it, you can imagine that small quantities will reflect that setup cost in the order.
The other way, tell us about your problem with memory and see if we can help. Frequently, forum members have contributed various methods of memory reduction that allowed some project to work within the given memory limitations.