A rubber wide top spatula works best. https://www.polytek.com/products/poly-paddle-mixing-tool
Drill mixing on a slow speed with a turbo mixing head is also good. Remember to scrape the sides and bottom of the container as well.
Wood paint sticks may contain moisture and are not very good for scraping the sides and bottom of buckets. Metal spatulas may shave off some plastic from the inside of the bucket and contaminate your mix.
Tongue depressors are fine for cup-size mixes.