Apple TV 4K vs Network Player for streaming via Air Play

I have all my CDs ripped to iTunes as 16/44.1 AIFF files. 

I want to stream them wirelessly to my stereo 

My current way of doing this is through my Apple TV 4K, connected to my TV via HDMI, with the TV connected to my stereo amp's DAC via optical cable. 

Thus it's iTunes -> Airplay -> Apple TV 4K -> TV (HDMI) -> Onkyo A-9150 (optical).

Is the sound quality suffering with the above setup? Does the Apple TV 4K introduce jitter (via HDMI to the TV)? Does it upsample my 16/44.1 music to 16/48 (or is that just for movies)?

Would an older Airplay-capable network player/DAC (like the Pioneer N-50) sound better than the above setup? In this scenario, it would be iTunes -> Airplay -> N-50 -> Onkyo.

Or are there other (better) options?
Do you absolutely need Airplay?  A streamer such as the Bluesound Node2i can do Airplay and connect to your amp.  However, the Node2i will also see the files
on your computer either wirelessly or by Ethernet.  You can connect to the amp by analog, using the DAC in the Node, or digitally, using the DAC in the Onkyo. You then can compare Airplay vs WiFi/Ethernet and decide what you prefer.
  I haven’t listened to Airplay critically, so I can’t tell you how it compares to the other modes, but my bias would be that it wouldn’t be as good as Ethernet or WiFi
For now, I have connected an Airport Express to my wireless network and to my Onkyo via optical, so I can stream music from my Macs to the Onkyo. Sounds pretty good and does not do any upsampling.

I might consider an Airplay-compatible network streamer in the future.

Thank you.
I believe all generations of Apple TV devices will indeed resample audio to 16-bit/48kHz, for all use cases. I haven't seen anything contrary to that yet.

Given your specific setup, I think it is unlikely you will get a different sound by swapping something else in for the Apple TV, unless the Onkyo reproduces 48kHz somewhat differently than 44.1kHz. In addition to the Apple TV buffering the audio data before transmission, the TV is going to be buffering it while it extracts it from the HDMI data stream, before outputting it via optical to the Onkyo.