H'okay, so I have a bit of a theory on why my lettuce died:

After looking at the sensor logs, a pattern I noticed is that the average temperature for the seeds was between 23-30 degrees celsius. The ideal temperature for lettuce seedlings is actually 16 to 21 degrees. So there's a bit of an issue there.

To mitigate that as a possible cause with the next test batch, I will likely leave the window open a bit to encourage slight cooling during the day to acceptable levels.

On that note, I will likely need to think of solutions for a window sill deployments up north. The window setup will likely need a temperature/humidity controlled environment