[Fire HD 8 and HD 10 Original Android Development] topher587:
I went ahead and installed it. It appears to be the same release. The build string is lineage_karnak-userdebug 11 RQ3A.210905.001 eng.kai.20211028.101703 test-keys. It uses opengapps pico instead of mindthegapps I believe.

Though, I'd try the following without reflashing, since it was actually not a total waste to keep tinkering this evening.

Went ahead and installed youtube vanced again, picked a random video on the front page, and let it spin for about 60 or so seconds where it actually did successfully begin playing YT content. This was the YT Vanced guide I used since it's been pulled. https://xdaforums.com/t/guide-how-to-install-youtube-vanced-post-shutdown-22-03-22.4419609/

As for Plex, I have some success to report there too.

I went into Plex and under the Help & Support section in the settings menu, I turned on network debugging. The error is related to the embedded Android Exoplayer video playback app.

08-19 00:39:47.323 e: [ExoPlayer][EventLogger] playerFailed [eventTime=7.21, mediaPos=4256.00, window=0, period=0, errorCode=ERROR_CODE_DECODING_FAILED
com.google.android.exoplayer2.q: MediaCodecVideoRenderer error, index=1, format=Format(0, null, null, video/avc, h264, 16084000, null, [1920, 804, 23.976], [-1, -1]), format_supported=YES
at com.google.android.exoplayer2.t1.handleMessage(SourceFile:43)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:223)
at android.os.HandlerThread.run(HandlerThread.java:67)
Caused by: com.google.android.exoplayer2.video.h: Decoder failed: OMX.MTK.VIDEO.DECODER.AVC
at com.google.android.exoplayer2.video.i.m(Unknown Source:4)
at v2.o.render(SourceFile:27)
at com.google.android.exoplayer2.t1.q(SourceFile:14)
at com.google.android.exoplayer2.t1.handleMessage(SourceFile:25)
... 3 more
Caused by: java.lang.IllegalStateException
at android.media.MediaCodec.native_dequeueOutputBuffer(Native Method)
at android.media.MediaCodec.dequeueOutputBuffer(MediaCodec.java:3452)
at v2.x.l(SourceFile:1)
at v2.o.drainOutputBuffer(SourceFile:7)
at v2.o.render(SourceFile:16)
... 5 more
]
08-19 00:39:47.332 e: [Player][ExoPlayer] Playback error detected
08-19 00:39:47.340 e: [FFmpegExtractor] Extraction operation failed with exception.
08-19 00:39:47.347 i: [Player][Timeline] Handling player error
08-19 00:39:47.348 i: [Player][Timeline] Playback stopped (error: true)
08-19 00:39:47.356 i: [Player][Timeline] Player error not known by server, reporting original player error (or a playback interrupted one)
08-19 00:39:47.356 i: [Player] Error reported: UnknownError (null)
08-19 00:39:47.357 i: [Player][Timeline] Playback stopped (error: false)
08-19 00:39:47.358 i: [TranscodeSession] Stopping...
08-19 00:39:47.358 i: [TranscodeSession] Session already stopped.
08-19 00:39:47.364 i: [Player][Error] Displaying player error...
08-19 00:39:47.365 i: [DialogBehaviour] Showing dialog.
08-19 00:39:47.366 i: [ExoPlayer][EventLogger] state [eventTime=7.24, mediaPos=4256.00, window=0, period=0, IDLE]
08-19 00:39:47.379 e: An exception occurred: com.google.android.exoplayer2.q: MediaCodecVideoRenderer error, index=1, format=Format(0, null, null, video/avc, h264, 16084000, null, [1920, 804, 23.976], [-1, -1]), format_supported=YES
08-19 00:39:47.380 e: Stacktrace: com.google.android.exoplayer2.q: MediaCodecVideoRenderer error, index=1, format=Format(0, null, null, video/avc, h264, 16084000, null, [1920, 804, 23.976], [-1, -1]), format_supported=YES
at com.google.android.exoplayer2.t1.handleMessage(SourceFile:43)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:223)
at android.os.HandlerThread.run(HandlerThread.java:67)
Caused by: com.google.android.exoplayer2.video.h: Decoder failed: OMX.MTK.VIDEO.DECODER.AVC
at com.google.android.exoplayer2.video.i.m(Unknown Source:4)
at v2.o.render(SourceFile:27)
at com.google.android.exoplayer2.t1.q(SourceFile:14)
at com.google.android.exoplayer2.t1.handleMessage(SourceFile:25)
... 3 more
Caused by: java.lang.IllegalStateException
at android.media.MediaCodec.native_dequeueOutputBuffer(Native Method)
at android.media.MediaCodec.dequeueOutputBuffer(MediaCodec.java:3452)
at v2.x.l(SourceFile:1)
at v2.o.drainOutputBuffer(SourceFile:7)
at v2.o.render(SourceFile:16)
... 5 more

08-19 00:39:47.392 e: An exception occurred: kotlinx.coroutines.b2: w0 was cancelled; job=w0{Cancelled}@667fa54
08-19 00:39:47.393 e: Stacktrace: kotlinx.coroutines.b2: w0 was cancelled; job=w0{Cancelled}@667fa54

I found a workaround for this within Plex though.

I installed VLC, completed the initial first-run wizard, and then from within Plex, I enabled External Player usage under Settings > Advanced > Player > Use External Player. After googling around about exoplayer, it won't use software codecs, but VLC can and will.

You don't get play status or marking as watched, but it's better than nothing at all. I'll have to look around to see if there are any other players. I've heard Kodi has integration with Plex, but haven't spent any time on it now that I have a workaround.

I'm satisfied enough at this point to keep the ROM.