I've only read the 6th and most recent volume and really enjoyed it. Šejić's art has certainly evolved through the project. That installment was an excellent standalone entry in the series and didn't require reading any previous material, so I'm curious to go back and see how the pieces all fit together. I believe Šejić has announced that Sunstone is planned for 20 volumes in total!
Here's the link to PDF volumes straight from Šejić that I found in articles from March. At that time Šejić announced that he was walking away from doing any further "corporate" work following the success of Harleen at DC, and was going to focus fully on his creator owned projects.
https://www.dropbox.com/sh/nneuv3k9es6k6ms/AAB5iscpij79U8j4826GcCMga?dl=0
It's an interesting business model that I've looked at for some time... giving your content away for free! Jason Brubaker was probably one of the first big proponents of it I was aware of. Both get staunch support from their fans who still want printed editions, back Kickstarters & subscribe to Patreon accounts to have better access and interaction with the artist.
Šejić is quite good at that Patreon thing if his number of patrons and comments on different tiers posts are something to go by.
https://www.patreon.com/shiniebezial
Yes, Volume 6 introduces new characters and could be seen as its own start again after 1-5. But Sunstone is heavy on continuity nevertheless.
!ENGAGE 50
ENGAGEtokens.