Game is submitted, still had some time on the clock but I'm on vacation now so I'm finished for now ;)
So here's how I progressed:
Day 1:
General fiddling till I figured out what I wanted to do. Settled on multiplayer shooter where players can shift their gravity. No idea why, just a random mix of things I wanted to try anyway. Got player movement, gravity shifting working in a cobbled together level and managed to get multiplayer working for movement.
Day 2:
Made selectable player avatars and got a pistol working. Got the idea that I could add some destructible objects to use more of the art pack. Started with the plants. Spent way too long on that, didn't get it to work properly.
Day 3:
Added more guns and damage for players. Made the plants only destructible by the splash damage of the newly added rocket launcher. Added health bars
Day 4:
Build a lobby server to register and fetch active games. Tried a connection outside by LAN for the first time. Should have tried that earlier...
Day 5:
Replaced godot's MultiplayerENet with the new WebRTC components. Worked good and with ultimately very little changes but took some fiddling and multiple tries due to brain farts. Tested for the first time with multiple players. Fixed some bugs, changed some guns and played for a while. Still missing lots of stuff but quite relieved it's at least fun to play.
Day 6:
Slapped together some comfort features like a stats screen, a way to disconnect from a server, kill messages, volume controls and some sounds. Fixed a few more bugs. And then played some more.
Still have a few features and ideas left for after the jam and some things still aren't working like I want them to but overall I'm quite happy for the time I put (and with it being my second game overall).
Not quite sure if multiplayer was a good idea for the jam, because it takes at least two players to play the game in a meaningful way. But it's done now so too late for that.