Hi there, I played your game in the the browser and it seems like a solid start.
I think a good place to start expanding would be by making a second enemy with different logic to the current chasing variety, with the goal of setting up a simple system so that adding more variety will be easier in the future. If you’ve never researched “state machines”, I’d advise looking into those since they are very simple to understand and code.
Path finding would also be useful, whether it’s for another enemy, or to make sure the chasing monsters are less likely to get stuck behind obstacles. Being able to use path finding will most probably be very useful for future projects.
After that, I’d probably start trying to work out how your upgrade/ability system works… Does it interact with the state machine? Is it a sub system within the state machine? Do they update per frame, or on specific instances? I think if you're able to loosely describe what you want, researching solutions will become easier.
I think that would be a pretty good start research-wise, since they are easy to find, understand and put together, but will give the game a lot more variety and be skills that you’ll use and refine in the future.
Other than that, I noticed that the browser version still had an “EXIT” option, which you usually don’t want for browser and mobile games since a web or mobile application isn’t expecting the game to terminate itself. In Godot, you should be able use the functions “OS.get_name()” or “OS.has_feature(tag_name:String)” to avoid having to code each version differently.
Hopefully some of this is helpful and best of luck with your future game development :)