Picked up a Synology DS1515 from a client who upgraded. NAS has been lightly used in an office environment solely for file sharing.

Intel Atom C2538
2 x 4GB Micron DDR3 PC3-12800S
NAS HDs are 5 x WD4000FYYZ-01UL1B1 Enterprise (Synology lists as compatible)
DX-513 expander - 5 x WD60EFRX-68L0BN1 Red (Synology lists as compatible)

DSM7.1-42661 Update 3
Docker 10.8.0
Jellyfin/Jellyfin Latest (not linuxserver)

iPad Pro 11" 3rd gen (M1) iOS 15.5
- 'Native video player (Beta)' enabled as well as 'Prefer fMP4 in HLS'

I spent a few days recently overhauling a new-to-me NAS with expander. All drives are in good shape, and approved by Synology for this model of NAS. I upgraded the RAM to some 1.5V Micron DDR-3 SO-DIMMs (first set I tried were incompatible as the were DDR3L.)

I did a factory reset of the NAS and went through and did basic setup on the network. I then spent some time fixing basic stuff like making the DSM admin port available on the LAN. I did a little digging and came across a couple of basic Jellyfin tutorials which pointed me towards installing Docker and then adding the basic Jellyfin container. I moved my media over from my Win10 install and then configured libraries and thought I was ready to go.

I watched a couple of 1080p movies, and had no issues, however a week later I fired up a 10bit HDR MKV file and couldn't get it to play on my iPad. I then fired up a 1080p H264 SDR movie and it took over 50 seconds to start playing on the iPad.
- DSM RAM Utilization ~25% CPU Utilization ~99%
- Jellyfin container RAM usage 3.4GB CPU utilization 94%

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I've identified three separate symptoms currently, which may or may not be linked:

1. When quitting out of a movie, RAM utilization and CPU usage will stay pegged at streaming levels for 15 minutes after quitting back to the Jellyfin Library menu on the iPad. If I restart the container, memory usage will restart around 200Mb but will ramp back up to 3.4Gb as soon as something is streamed. I would expect the CPU utilization to drop back within maybe 10 seconds of the movie no longer streaming - is this normal?
2. I'm guessing that transcoding is not working, but I don't know whether it's a driver issue or a configuration problem. I've tried to follow tutorials such as this one or this one here for enabling hardware transcoding, however the video is no longer available, and my brain melts when i try to follow the instructions (Veteran windows sysadmin, but linux n00b)
3. I have a bunch of errors coming up in the Jellyfin logs when I stream. This log snippet was from the slow but successful stream of 1080p material:

https://preview.redd.it/008wrlmmjsc91.jpg?width=1613&format=pjpg&auto=webp&s=6f46d6fc2a2a8cd589e79d9e68144abe07801343

My questions are:
- Is my install damaged
- Given my platform, would I be better with the Linuxserver/Jellyfin container?
- if I convert, will I lose all my libraries or, worse, have to move my media (guessing not?!?)
- With my NAS running an Atom, is that compatible with any form of transcoding?
- Should I just allow my iPad to handle and process a raw video stream? It should be powerful enough.

Thanks in advance for any and all assistance.