Ventoy to hdd partition

[KaMyKaSii]

Yes, well, but then I would have to get a whole new HDD (or sacrifice one of mine) just to do that.
Whereas, if I could specify the partitions myself, I could just use one I already have (with things on it).

Ventoy needs to be installed on the entire device because it needs to install its own bootloader. If you just want to install Ventoy to a disk without needing to format it, follow this tutorial by me. It was made for MBR so you should check the structure differences with GPT to adjust the dd commands. But no, you won't be able to boot operating systems installed on partitions on this same disk, I don't say it's impossible, but at least not in a simple way. You may be able to clone your current systems into VHD files and boot it with Grub2 File Manager, but I'm not sure

I, too, have desire for being able to place my /BOOTISOS (the folder where I keep all of my Ventoy ISOs on the 1st partitiion) anywhere on system's storage and to have Ventoy search and allow selection from that folder of ISO contents.

And you can do this. Use Ventoy to load the Grub2 File Manager (grubfm_multiarch.iso), and then use it to boot your ISOs from internal storage. Just don't be stupid to format / delete the partition where the booted ISO is, of course