You should be able to direct play / direct stream multiple streams at one time and also transcode multiple 1080p or lower resolution streams.
The DS1520+ can transcode some 4K video, but it cannot tonemap HDR to SDR (*).
In Settings → Transcoder, make sure you have hardware accelerated transcoding enabled and HDR Tonemapping disabled. See screencap below.
Be aware that all Celeron CPUs struggle with burning subtitles, which occurs on the CPU, so avoid subtitles that force a video transcode (ex: ASS subtitles on most clients).
You can monitor playback via Plex Dashboard → Now Playing. It will show you if the video or audio is direct playing, direct streaming, or transcoding.
(*) The issue with tonemapping is that the necessary Intel drivers are not available in the Synology OS. You can tonemap HDR to SDR if you run Plex Media Server in via docker. See Synology FAQ 28.
