I use Qobuz. They have options for high quality flac downloads. Their download app isn’t native Linux, but you can download one by one from the web.

Try gamescope with the —hdr-enabled launch option

HDR is enabled in Plasma? Does your desktop look washed out when it’s enabled as well?

What launch options did you use for gamescope?

Host on your own hardware to avoid server fees. Use a free dynamic dns service, I used duckdns for a few years before getting my own domain. Are there other fees I’m forgetting?