Emissive quads are possible with a custom IBakedModel, a TESR isn't needed here. You would need to create and load a custom IBakedModel and generate a list of custom BakedQuads that have their lightmap data set to anything apart from 0, say 240 for full emission. An example can be found here.
I looked through the changes for 1.13 for the first time today, and I saw they are reworking the way items and blocks are implemented, along with other changes and additions like data packs and removal of metadata.
Will this mean a lot for me as a forge modder? I can assume that things will break, but on what scale?
Use the minecraft:item_nbt ingredient type. Set the nbt property to the NBT data you want.
I don't have any examples of the minecraft:item_nbt ingredient type, but you can see an example of a recipe output with NBT (which uses the same properties) here.