First of all, I take into account my client's desire for whether they want it to gradually disappear (thinning overall) or whether they want to clear it in sections that keep it somewhat masculine looking. Clear cutting areas is definitely faster than thinning since I'm not wasting time figuring out what hair I want to do next.
That said, for a particularly thick beard, I'll usually start by thinning it out until the density is low enough that I know I'm not going to damage the skin beyond recovery. Even then, a lot of my trans clients like to be really, really aggressive, with some doing as much as 4 hours per week to get started. When their skin reaches the point where I'm not seeing enough recovery from one week to the next, I force them to take some time off. I'll even clear an entire upper lip in one sitting.
You'd be surprised what the skin will recover from as long as your insertions are good, you use the right equipment and settings (for you - everyone's techniques are going to be a little different, some people are better at some modalities than others, one machine isn't necessarily better than another if you're comfortable with your machine, etc), and you pay attention to how the skin is reacting.
Also, don't worry about growth phase. Hair can die in any phase as long as you can see it. Telogen is a great time to kill hair and the schools that teach anagen-only are doing everyone a disservice. Schools also tend to be overly safe in their treatment recommendations to limit their liability and to make insurance companies happy - experiment carefully as you gain experience and confidence.