I am a hobbyist GMS:2 user, with aspirations to make a game good enough to publish some day. Every game jam I participate in improves my understanding of the tool and helps me understand the system better. This game was no different.
Something I've seen before but never think about until I'm packaging is that HTML5 does not always behave exactly as the windows version. My opening screen had to be replaced with the screenshot of the Windows version which rendered correctly.
Experimenting with movement methods, vs using a tried and true method, wasn't the best idea. As I progressed in the coding I realized I hadn't always taken into account the interaction between collision masks correctly, which led to quite a bit of rework.
Almost every previous game jam I've worked on has been a single room game and I realized pretty late I don't have a strong grasp on room changes.
All in all this was a great experience. --I just wish I'd had had time for sound and music.
--Have a great day and good luck!