Turns out that, if all you really want is the "Mini Windows 10" environment, then you can drastically cut down the used disk space all the way to just 1 or 2GB.
Basically all you need is the "Mini Windows 10.wim" file (which you can put right in the root of your disk if you want rather that in sub-folders) along with the entire "USB:\ventoy" and "USB:\system" folder.
From there, if you want any PortableApps, copy over the files "USB:\start.exe" and "USB:\CDUsb.y" as well as all of the
files (not folders!) located directly in "USB:\PortableApps" and also copy the "USB:\PortableApps\PortabeApps.com" folder too, and then copy over whatever program folders you want from "USB:\PortableApps" (making sure to not change any of the folder paths)
Lastly, if you want any of the non-PortableApps programs (e.g. 7-zip, Firefox, or Macrium Reflect), then copy over the files "USB:\Programs\Software Lister.exe" and "USB:\Progarms\Softwares.txt" and then also copy over whatever programs you want from "USB:\Programs" (again, making sure to not change any of the folder paths)
Do note that there are at least some default file associations that expect Chrome and MPC-BE to be present in "USB:\Programs", but since it seems Firefox isn't set for any associations, this means you can just as easily add the most lastest version in PortableApps form instead (and oddly enough, IrfanView non-portable is set up as the default image viewer despite not being present in B:\Programs... but you can add it yourself by downloading the latest PortableApps version of IrfanView and copying over its "App\IrfanView64" folder in USB:\Programs\ and then renaming it to "IrfanView_x64")
EDIT: Holy crap I just discovered an awesome protip: If you put the "USB:\start.exe" and "USB:\CDUsb.y" onto another partition (I had it as NTFS, but no idea about other formats), then you can actually store the "PortableApps" and "Programs" folder onto that other partition and yet have things "just work" (I had it on the same disk, but no idea if that's actually required). The main benefit is for running live Linux ISOs since, by default via Ventoy/MediCat, they typically can't access the partition the ISO is stored on, but having your programs and data and stuff stored on another partition means that those live Linux ISOs
can actually access them! (and for those that don't know, you can actually install Wine during at least a live ISO session of Ubuntu or Linux Mint, but you'll need like at least 12GB of RAM).
And in the process of this, I noticed that only the non-PortableApps version of WizTree is included, yet the PortableApps version of WinDirStat is included? (and it's my impression that WizTree should just be all-around newer and better than WinDirStat). Perhaps a PortableApps version of WizTree didn't exist at the time, but here's the link to it:
since medicat's windows is booting from a WIM file you can do it, just follow something like
https://www.tenforums.com/tutorials/95002-dism-edit-registry-offline-image.html
Thanks, but honestly that seems a bit involved... I might take a gander at it eventually some day (maybe at the same time I finally try taking a look at a forked version of HashCheck Shell Extension that re-enabled MD4 support).