So just to clarify when you say completionist do you mean;
- Build every single item available
- Build every peice of equipment available
- collect/make all possible collectable items
- max level every peice of equipment
- max level all workstations
- cook and eat all possible food items
- max level all skills
- kill all bosses
- listen to all available boss dialogues
- collect all trophies
- kill every type of enemy plus star variants
- tame all tamable animals
- breed all tamable animals
- buy all items available from Haldor
- discover all biomes

Did I miss anything?