I'm right handed, with big enough hands I've never had an issue manipulating the safety/decocker with my right thimb. That said...
To rack the slide whether it's loading, an immediate action dril, whatever, I rotate the pistol on it's bore axis slightly inboard and slingshot the slide. This keeps you from accidently putting it on safe as you would if you overhand the slide. The key is not overhanding the slide, grasp it between your thumb and forefinger only. This was the preferred method to mitigate that problem when transitioning folks who were accustomed to SIGs or Glocks to the Beretta on a job I worked overseas several years ago. Failure rate was minimal, especially with proper practice.
Keeping it decocked and off safe ("lever down, lever up, reholster" was our mantra) is great in theory, but only if you use a holster that covers the safety/decocker, because in practice my observation has been the it wll be swiped on, off, and not where you left it, by clothing, gear, seat belts, etc. As a result, I just got in the habit of flipping the safety off as part of my draw stroke just like I would with a 1911.
Again, I never had a problem using my thumb to do this. BUT, others did, and we taught them to use their left hand thumb to manipulate the safety/decocker once a firing grip was established as opposed to a single handed grip and manpulating the safety/decocker like it's a switch on a dashboard.