I bought a set of diamond core drill bits on Amazon and it made adding drainage holes waaaay easier.
They were not expensive, either under $20.
Once you learn the technique (I watched YouTube videos) it's super easy with the right tools.
Edit, these are what I bought if it helps, hope that's okay to post: https://www.amazon.com/dp/B077K31QT2/ref=cm_sw_r_cp_apa_326mBbDK27676