Yep block placing and breaking is already supported, but light currently don't so everything is dark :/, We are working on it. Yep, We want to add all cool features like scoreboards, teams and we already have a API which is similar to use to Brigadier for our Commands.
Thanks, I would love to give players on low-end hardware the possibility to host servers. I think i may test Pumpkin on a raspberry PI or something one day
Mojang made large improvements to the lighting engine in 1.20, bringing it in line with the performance mods Phosphor[1] and Starlight[2]. Despite being deprecated now, they might still offer some useful insight into how to approach such a system in a performant way. You'll need to be mindful of the licenses, but it's likely easier than reverse-engineering Minecraft (even with mappings).