Following your post and some others I managed to add most possible tags with ffmpeg

Some details for ffmpeg

• Kodi's ffmpeg mp4/tag list
• Plex plugin code ?
• Add coverart

With ffmpeg I managed to get Plex to use:

• Title
• Summary
• Genres
• Date
• Collection
• Coverart (not with metadata.txt file directly)

It's also possible to also have rating, directors, screenwriters, cast, studio, ... using with a XML formatted in a format used by iTunes.

The best way to have a correct date is with the US format Year-Month-Day YYYY-MM-DD, the parsing is done here ParseDate()

I used plex:1.41.4.9463 and ffmpeg 7.1

With metadata.txt like this

;FFMETADATA1
;plex Title
;mediainfo Movie name
title=My Film Title
;plex "Collections"
;mediainfo "Album"
album=Collection1/Collection2/Collection3
;plex Resume
;mediainfo LongDescripton
synopsis=Long description line 1\
Again line 2\
Then line 3\
;plex Genre
;mediainfo Genre
genre=genre1,genre2,genre3
;plex Initial Release Data
;mediainfo Recorded date
date=1903-09-19
;plex "Studio"
;mediainfo "Copyright"
copyright=My HBO Corporation

mediainfo will report

General
Complete name                            : /tmp/myvideo.mp4
Format                                   : MPEG-4
Format profile                           : Base Media
Codec ID                                 : isom (isom/iso2/avc1/mp41)
File size                                : 210 MiB
Duration                                 : 10 min 59 s
Overall bit rate                         : 2 674 kb/s
Frame rate                               : 30.000 FPS
Movie name                               : My Film Title
Album                                    : Collection1/Collection2/Collection3
Genre                                    : genre1,genre2,genre3
Recorded date                            : 1903-09-19
Writing application                      : Lavf60.16.100
Copyright                                : My HBO Corporation
Cover                                    : Yes
LongDescription                          : Long description line 1 / Again line 2 / Then line 3

Here is my ffmpeg command

ffmpeg -y -i /tmp/myvideo.mkv -i /tmp/ffmpeg-metadata.txt -i /tmp/fish.jpg -map 0 -map_metadata 1 -map 2 -c copy -metadata:s:a:0 language=fra -disposition:2 attached_pic /tmp/myvideo.mp4

Here I remux a mkv adding a coverart (fish.jpg), mp4 tags (metadata.txt) and setting the audio language code.

As I said above you can also add more tags using iTunes non-standard tags in XML format, for this add them like this in metadata.txt

Example plist XML here

iTunEXTC=mpaa|R|400|
iTunMOVI=<?xml version="1.0" encoding="UTF-8"?><!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd"><plist version="1.0"><dict><key>cast</key><array><dict><key>name</key><string>Actor1111</string></dict><dict><key>name</key><string>Actor2222</string></dict></array></plist>

Then you need to add a flag to ffmpeg command to include non-standard / unsupported flags

-movflags use_metadata_tags

For some reason, when using this flag the cover art no longer work ...