Did this have actually have an ending/can it be beaten? I got stuck at a certain point with no apparent way to advance the plot
See below, if you've gotten that far, you should just need to hit play one more time from the main menu, and the ending/credits will be triggered.
If your experience doesn't match up, let me know and I will see if I can find a bug around where you are.
ENDGAME SPOILERS!
After the "new" Frizzle shows up, there should be some dialogue between Frizzle and the narrator, after which the narrator will tell you to crank and delete Frizzle. You can either crank, or not, and the game will "crash" and send you back to the title screen.
After this, hit play one more time to trigger an ending based on whether you deleted Frizzle or not, and the credits will roll shortly after.
I tried again, and saw the frizzle clone 2 or 3 times (not sure since I had it on standby for a few days), got kicked back to the start screen, and the playdate crashed when I tried to start the game again. Whenever I saw the frizzle clone, special dialogue didn't pop up. The last time I saw it, I got a prompt to play the simon says game, and it was gone after the minigame ended.
Thanks for continuing to play and sending me details. What you're reporting sounds like a bug, so I will have to see if I can reproduce the issue to troubleshoot.
I am thinking it may have something to do with going idle, and then returning to the game. I'll let you know if I am able to reproduce this on my end.
I have been testing but unfortunately, haven't been able to reproduce the behavior that you're seeing. I found a few specific actions that can sort of sequence-break the game, but they don't prevent forward progress. I'm sorry you haven't been able to get the intended experience, I will let you know if I do manage to track down and fix the root cause, but I won't be offended if you request a refund.
At this point, I think you've hit some kind of edge case that got your game saved in a broken state. If you haven't tried already, and you want to give it a fresh go, you can try deleting the data via the Playdate settings to start completely new. Settings > Games > Frizzle > Delete Game Data.