This is a great subject. I am still coming across new features in Doom Builder 2, Slade, the Doomwiki and word of mouth. Which I learned quite a bit in the past year and prior.
I gotta say I have built confidence when it comes to ACS scripting. I felt intimidated when I first started working on Jazz Jackrabbit Doom (2018). I always avoided using any scripting but with the type of mod I am working on, the map definitely needs it.
Lumps on Slade. ANIMDEFS, Terrain, Mapinfo, etc. I use to screw up pretty bad if I left out something. Now, it has been easy to understand and not to stress about.
Designing maps with lots of detail and length. I used to design short maps, you can beat in under a couple of minutes. Now, it will take time to go through.
I want to learn how to work on my own midis. As a musician, I should know this, which is silly to me that I haven't learnt it. 🤣