Lets start by saying its NOT "my" zip, its simply a zip I found on here, somewhere, which works.Not sure that I understand. Your zip file switches whole SElinux to permissive which is a security risk and some apps will detect it and refuse to launch. "Mine" fix (the idea is included in Viper Magisk module, but it doesn't work with recent Magisk versions) patches only necessary SElinux permissions.
Legacy mode is required for some apps (e.g. Youtube or Netflix) or system wide audio processing. New "non-legacy" mode attaches to audio sessions which are usually provided only by music player apps. Legacy mode switch is just above the compatibility mode, at least on my phone.
The SELinux is only for the first 3 minutes of boot (and could be cut down). On my Pixel 4a, this is not a big risk, as the phone takes a while to get to a usable state. Even the wallpaper does not load until after you first log in. And that time could be cut down or possibly removed. Thats what I was explaining in my post.
You talk about "legacy mode" But I have only seen "compatibility mode" in the viper menu. Where is this "Legacy Mode" setting?
I just checked, and Viper IS applied when I play YouTube videos, with compatibility mode turned off. I am using Vanced for YouTube.
Having a post-fs-data script should ONLY be used if placing it in service.d does not work.
The zip applies the same SELinux allows as your post does, but its in service.d, not post-fs-data
By 'recent magisk versions", which are you talking about? I am running 22.0 on a Pixel 4a running March A11 firmware, but have seen a very recent magisk update of 22.1 is available, which I was planning to update in the coming week, along with my ROM to Aprils release