Python...

So I can confirm that you are incorrect.

Having installed Python 3.11 fully, and changing the PATH variables, Plex breaks and no longer plays video (since the mpv dll is trying to do something it cannot, Plex just crashes) This is because the mpv64 plugin is no longer where it looks for python and the plugin files, since it then looks at the python folder...

When I copy the contents of the mpv64 folder into the python folder, Plex works properly again. And then I have the full use of python and its plugins... To which now function as to be expected. (I'm not going to waste time trying to identify exactly which files are needed in that folder since that's a huge waste of my time. But it does in fact work to run SVP and mpv64 from the python folder)
In this case it was the folder located in appdata/roaming/python/python311
PATH is now set to that location for python. And the contents of the mpv64 folder are now in that folder to accommodate compatibility.

OP is completely correct.
I've had this exact issue for years.
I like and use and paid for SVP, so I don't want to through away the money I spent on the software...

But it completely destroys the ability to use any other Python ... anything.

I can't believe it runs this way. I've never encountered any other piece of software that completely hijacks Python for its own purposes and prevents anything else from working with it.

I wish I had answers...

Okay... but why even make the default action do that in the first place?

Well, are you not saying that if you don't want SVP to take control of your Python path that you have to install something externally?
Vapoursynth?

I'm trying to understand how you think this happens.
I've just tried doing what you said, made sure I unticked everything except for the core application during a clean install.
It once again destroyed my Python install.
So I'm really unsure how you think this is acceptable.
Would love for you to write a small guide on how to install SVP without it ruining Python on your PC.

It 100% does... I don't know why you keep thinking it doesn't. It ABSOLUTELY modifies PATH.

This is the environment variables after a clean install with all default settings.

Are you not actually a developer of the application?
Do you not know how it works?

I've just encountered the same problem. I did the same thing as you to no avail - Plex was still crashing. I figured out a way, though -- a .bat file which will run Plex for me with some custom environment variables set, therefore not messing with my global Python installation.
Please note everything is hardcoded, but it should work on standard installations. If you use HTPC, you might need to modify the path, but it should be pretty readable. It won't let me attach a bat file, so I'll just include it as text - hope that's fine.

@echo off
setlocal

:: Get the current PATH
set "originalPath=%PATH%"

:: Create a new PATH, excluding directories containing 'Python'
set "newPath="
for %%P in ("%originalPath:;=";"%") do (
    echo.%%~P | find /i "Python" >nul || (
        if not defined newPath (
            set "newPath=%%~P"
        ) else (
            set "newPath=%newPath%;%%~P"
        )
    )
)

:: Add the new directory to the PATH
set "newPath=%newPath%;C:\Program Files (x86)\SVP 4\mpv64"

:: Set the PYTHONPATH
set "PYTHONPATH=C:\Program Files (x86)\SVP 4\mpv64"

:: Run Plex.exe with the new environment
cmd /C "set "PATH=%newPath%" && start "" "C:\Program Files\Plex\Plex\Plex.exe""

endlocal