Not sure if this is what you are trying to do but this is what I do. On my 64 gb USB install Medicat with image USB as normal then use mini tool partition 12 (on medicat) to extend the medicat partation by 10gb to give me more space to add iso's if needed. This still allows you to update ventoy if new version is released by downloading it then clicking on update.
The problem that I am facing only happens on a small percentage of computers 10% of the computers that I test on and they are a lot.
On those 10 computers my 256 GB flash disk does not work and shows a GRUB error Mini Bash prompt although the same flash disk works normally on others. I noticed that this is related to partition size (Just on those computers) . When I create a fresh ventoy on my large flash disk it gives the same error so I know it is not a MediCate problem it is a ventoy problem. When I burn the image on the flash disk it works normally but as soon as I move the ventoy partition to the end of the disk (even without resizing the 32 GBs partition.
So the problem as I noticed is on some computers only when the ventoy partition is moved to the end of the flash disk.
I repeat it is not a medicat problem and only happen with some computers 10% with the same working large flash disk.