I can’t play your game (it’s Windows-only) but having submitted to 23 of the 24 jams I’ve joined, I feel like I can comment on actually finishing things.

In general, you want to know what the “core” of your game is. What is the thing you want the player to be doing most of the time? Start with that, with no real assets. Personally I’m often making movement-focused games, so I’ll start by making just the physics in empty rooms with boxes for characters.

At that point, you’ll either have something that’s already fun, or you have nothing. Make a snapshot. (You are using version control right?) From there, you can add features, but always make sure you have a playable game. Snapshot frequently, basically whenever you get the next playable thing. If you have a list of “NEED THIS” and “NICE TO HAVE”, focusing on the needs first, you’ll be fine.

When time runs out, submit that latest playable version, that’s your game!

All that is on planning. Estimating timeframes is always going to be significantly harder. That’s the kind of thing you can only get with experience, and even then the typical recommendation is to double or triple everything you imagine. For instance, in my submission I have an achievement system, and I was sure that I’d be able to fill out a list of 48 achievements in a few hours. But the reality was significantly beyond that, and as a result some things just didn’t happen. This is fine. 72 hours is not a lot.

Personally the way I do game jams is to have big ideas and then from there start saying no to myself! The reasons can vary but my most common ones are that it takes too long to implement so I say no or that the core of the game is still there if I don't do this feature. Once you have your core you want to make the system as unsophisticated as possible, make it simple and easy to implement while achieving the same goal as if you had developed something more complex.

The game I submitted for this jam (called From Beyond the Wall) had a good amount of "no don't implement that". My plan starting out is to have a much longer level and a real time lighting system but after starting to develop it and seeing how much time I have left I just said no to the two of those and went with a shorter level and the light in my game is basically just a circle collider that turns on or off every few seconds to determine when the monster can or can't go through. Speaking of the monster, it's literally just an object that moves towards the player with minimal AI that interacts with the 'light'.

As for planning I can't help you there, my workflow is too sporadic and chaotic :(