Do you have recordings of the two issues?

This is the pop or snap sound. Sometimes it's just one pop, sometimes it happens in twos. Can be a few seconds apart, or a couple of minutes apart until the system "warms up".

Here's the other noise when playing music from spotify over the network. You can hear a slight crackling in the music. Sometimes it's more extreme than this, with the music only comes through in little bursts.

Sorry...I recorded these with my phone, so they aren't great quality.

Check the buffer size you're using on the iMac.
I was running a test last week with Studio One feeding 64 channels through Dante to my A&H system and got horrible crackling that I spent ages searching for. After changing network switches and cables I checked the audio setup in Studio One and increased its buffer size from 32 to 64 samples and all the crackling went away. If its an old or low power iMac you may need to increase buffer size to allow the cpu time to process the audio before sending it via Dante.

As for the occasional "pop" from your speakers... What network switch(s) are you using? Dante doesn't like EEE (power saving feature) built into some consumer grade switches. Again I had this problem, its caused by the power saving momentarily turning off the network port between data packets and not turning them back on fast enough to prevent data loss. It doesn't matter for normal networking as the packet would simply be resent and you never notice, but with streaming audio packet loss means silence, or pops.
The answer is to ensure your switch does not use EEE, or use a managed switch where you can turn the "feature" off.

Good luck.