First of all congrats and thanks for this project it looks great.
I’m currently using SmartThings as a bridge to my ZWave/Zigbee/Wifi things, I also considered using HomeAssistant but the Google Assistant support skewed my decision.
It seemed it would be pretty hard (read impossible) to get a small hobbyist smart home app reviewed and accepted by Google (it would also require me to configure remote access to my bridge and maintain some DNS records).
What Samsung does with SmartThings is acting as a bridge to the Google smart home services. You can enable the SmartThings access inside the Google Home app and then your devices become visible from google home/assistant.
SmartThings also comes with severe limitations when it comes to extensibility and I would love to jump ship to your solution. Would you consider providing a solution to bridge “Project Things” to Google Assistant in a easy way? It would certainly be a bit more involved then the remote access you provide, but I think the “cloud” server could remain relatively simple, it would just redirect traffic to the hub and a plugin would be responsible to handle the heavy work.