When casting to Chromecast, the Tidal and Chromecast volume controls are tied together and don't operate independently. Chromecast pulls the stream directly from the Tidal server, and volume in the Tidal app is like a remote control of the Chromecast.
When using Chromecast Audio's optical output (connected to an external DAC), set volume on the Tidal app and Google Home app to maximum, to best preserve the digital stream sent to your DAC. Then control the volume using only your amplifier.
If connecting Chromecast Audio's analog output directly to an amplifier, there is a possibility that the output of the Chromecast may clip during loud passages with the volume turned up. Set the Chromecast volume to a level somewhere in the middle where you can get a wide volume range using your amplifier's volume control. Basically set the Chromecast volume so that its output is approximately like a line level signal expected by the amplifier.