For Komga with a 2TB manga and visual novel library and two or three readers at the same time, eight gigabytes of RAM is already enough. Komga is not memory hungry at all. If you use Docker it is still fine. Sixteen gigabytes is only worth it if you plan to add more apps later.
Your bigger concern is the two simultaneous 1080p transcoded streams. That depends entirely on the CPU, not the RAM. Intel CPUs with Quick Sync always handle this more reliably than AMD. Many AMD CPUs can do one h265 or h264 transcode in Jellyfin, but two concurrent jobs is usually too much unless you use a newer Ryzen with a strong iGPU.
If you want the safest pick, go with an Intel based NAS. Here are the realistic options in your budget.
QNAP TS 262
https://www.amazon.co.uk/s?k=QNAP+TS-262
Two bays with Intel Celeron N4505, Quick Sync, good for two 1080p transcoded streams, and supports NVMe SSD.
Asustor AS5402T
https://www.amazon.co.uk/s?k=Asustor+AS5402T
A bit more powerful with Intel N5105, also good for transcoding and has dual 2.5GbE.
Synology DS224 Plus
https://www.amazon.co.uk/s?k=Synology+DS224+Plus
Very easy to use, supports Docker via Container Manager. Transcoding works but not as strong as QNAP or Asustor for h265.
Regarding NVMe cache, Komga does not benefit much from caching because your files are large and read sequentially. You only need NVMe cache if you host lots of small database files or run more apps. For your workload, HDD alone is fine.