Thanks for playing the game, I glad you enjoyed it. Overall this is fantastic feedback.
The descriptions could certainly be more informative, the effects themselves are about +10% in most cases, less in others. I'll look into making them a little stronger.
Overall it sounds like you've found and mastered a style of play I hadn't thought of, and I like it. However the down side (or I suppose 'consequence') of staggering the time slow effect the way you did is just as you discovered, increased invulnerability on enemies.
So funny thing about the invulnerability is that it only lasts for 30 milliseconds at normal speed, that's less than a 3rd of a second. But if you are frequently slowing time it can last up to 3 seconds since its affected by time's speed just as with the enemies themselves.
The idea was to both discourage spawn camping but also to encourage players into dropping out of bullet time as enemies spawn, then slow it down again when going for the kill. I'll look into the timings, but can't say I'd reduce it by more than a few milliseconds, 25 milliseconds or 1/4 of a second might be a reasonable compromise, I'll give it some thought.
As for time spent making the game, I didn't learn about the jam until there were only 4 days to go. I'd just reached a milestone on a long term project and was perusing the upcoming jams for some fun when I saw it. Overall I spent the first day deciding if I wanted to join so late in whilst brainstorming some ideas before spending the remaining 3 on actual development. Had I had the extra time, it definitely would have gone towards increasing and balancing the boon/curse pool, more enemy variety would have been at the top of the list of additions too.