Wow this is great info. I aspire to do better in jams myself - would you be willing to share what your plan looked like? I should do this!
It wasn't that in depth. It just helps keep your mind on track and in scope.
Basically it was a list of days, 1 > 10. Each day had 1 or more tasks. Day 1 for example, was fully dedicated to art and getting a full mockup of what the game would look like, and a story summary. Art first is particularly helpful as you might hit little issues you didn't forsee, and it's far less work to correct little art issues than code/implementation issues (which can often involve art changes too). I also listed a bunch of art assets here, just to help remember and it's like a little reward every time you tick one off. Day 2 was the game "shell"; the splash, menu, options, cutscene system and the most basic game world, all connected. This can often be a rush job at the end, so I figured doing it up front would mean it'd get as much polish during dev as the game itself. Day 3 was about the importing of levels from the Tiled level editor, collisions and avoidance for the squad. So this included world collisions, projectile collisions, etc. And the game was playable at this point. Each 4 days after that was dedicated to a single level per day; new enemies, the boss fight, etc. The last 3 days were for general play testing, fixes and polish.
I know it's not a great guide. But having a small, simple, concise plan really helped me. Compared to what I usually do which is "I'll do this... Maybe tomorrow, it's fine... OK, the day AFTER that... It's all good... Ah crap 1 day left!" :P