I think WackyToaster has a great suggestion to make something like an idle-mmorpg as a starting point. Keep the scope small, so you can complete it. Something that does infrequent updates, does not require detailed game state to be synced, etc.
_After_ that, think about something more complicated, again I would keep the scope small.
We have implemented a MMORPG using C3 for client, with Spine for animation and skinning of the character and a lot of Javascript/Typescript on the client side to sync state with server.
The server is node.js. Does game logic, interface with db, auth, etc.
We use Colyseus for the networking/rooms (server and client).
This all took a _lot_ of work and iterations by our team. It is complicated.
Colyseus info: (we don't use the C3 addon ourselves, we use the TS/JS client library instead.)
docs.colyseus.io/getting-started/construct3-sdk