Alright, here's what I've done:

First of all, I figured out that the reason why I couldn't change the permissions was because I had created the thing using the Windows version of Ventoy (back before I had realized that a Linux version exists), so I just made a new one today using Linux. I did it like this:

sudo sh Ventoy2Disk.sh -I /dev/sdb
umount /dev/sdb1
sudo mkfs -t ext4 /dev/sdb1
sudo e2label /dev/sdb1 ventoy
cp -p [Mint ISO] [USB partition]

So this enabled me to create the Ventoy drive, reformat the manually editable partition into ext4, relabel it "ventoy" (because the label got erased during reformatting and I didn't know if it needs to be called that), and copy the ISO file onto the partition of the USB drive. This all worked fine.

Then after that, I wanted to make it persistent and also have it load the whole ISO into RAM when it boots, so I ran:

sudo sh CreatePersistentImg.sh -s 4096 -t ext4
cp -p /[path]/persistence.bat [dest path/file]

Then I made a folder called "ventoy" on the partition, and a file inside of it which is called "ventoy.json" and contains this:

{
"auto_memdisk": [
"/linuxmint-20-cinnamon-64bit.iso"
],

"persistence" : [
{
"image": "/linuxmint-20-cinnamon-64bit.iso",
"backend": "/persistence.dat",
"autosel": 1
}
]
}

The persistence should work because it's the same way that I did it before, except that I added "autosel": 1, which causes it to choose the persistence option by default, bypassing the choice to be able to run it without persistence. The memdisk seems to work because now it says that it's loading the ISO into RAM just after I choose to boot Mint but while it's still in Ventoy.

When I booted it, at first I noticed that my boot menu only shows the partition that can actually boot it (when I had created it in Windows it showed both partitions, not that the other one needed to be listed, but it's just a notable difference).

Then when it tries to start Mint (using compatibility mode, as I always do for the installer), it starts off fine and normal, but it gets to a point where there's an error line that repeats numerous time and fills the screen:

/init: line 49: can't open /dev/sr0: No medium found

I'm not sure what /dev/sr0 is or why it can't find it. I mean usually I get /dev/sd something and it's a drive. Anyway, after if finally finished displaying that error hundreds or thousands of times, it said:

BusyBox v1.30.1 (Ubuntu 1:1.30.1-4ubuntu6) built-in shell (ash)
Enter 'help' for a list of built-in commands.

(initramfs) unable to find a medium containing a live file system

Now if I had to take a wild guess about what all of that means, having no real idea, I'd say that "initramfs" is an abbreviation for "initialize RAM file system" (am I right?) and I guess that would mean that /dev/sr0 refers to some drive being emulated within RAM. So if that's all true then it means that it's having trouble getting the data that it needs to use for that, which means that the problem is being caused by my "auto_memdisk" in the .json file, meaning that memdisk isn't working for some reason. Is that accurate?

Why wouldn't it be working? I was really hoping to use memdisk mode, because the thing is - and correct me if I'm wrong - since I can't seem to enable persistence for loading in already stored data without also enabling persistence for storing new changes, but I think the changes only get made when the computer is told to shut down or restart, then theoretically, if I unplug the USB drive after it's finished booting and have it unplugged when I shut down (or perhaps even just push the button on the computer to shut down or restart, since there's nowhere to save any changes, anyway) then it should be prevented from ever writing any new information after that point, right? So I just thought that it would be best to have it load everything into RAM in the beginning so that the USB drive could be removed.

EDIT: Hmmm, I just read something that said it might work better if I put it into a USB 2.0 port (though I'm afraid that might be terribly slow!) or make the partition less than 4 GB (though Ventoy seems to want it to be the rest of the space other than what it uses for itself, and I'm currently using a 4 GB .bat file), but I'll try it in USB 2.0 and hope for the best.