Etching PCB - dealing with anhydrous ferric chloride
$\ce{FeCl3}$ ( and similarly $\ce{AlCl3}$ ) has very different properties, comparing its anhydrous and hydrate form. They cannot be easily transformed one to the other. Heated hydrate decompose to the oxychloride or oxide eventually. Anhydrate put in water undergoes violant hydrolysis.
I suggest obtaining the hydrate, or preparing it yourself, e.g. reacting of iron, $\ce{HCl}$ and $\ce{H2O2}$ ( this mixture of the 2 reagens was an alternative Cu etching solution, when $\ce{FeCl3}$ hydrate was not easily available in the communist Czechoslovakia, but is more aggressive then $\ce{FeCl3(aq)}$ ).
Another option is to let react the anhydrate with limited water outdoors and that adding concentrated HCl. For the dissolving itself it would be the best to put in directly to the acid:water 1:1, but it would develop dangerous fumes.
Avoid mixing concentrated stuff, it would turn into a chlorine chemical weapon. Start with 10% peroxide and 1:1 acid, calculate the ratio. Tune it later at will. Keep the etching solution rather mild than strong, to simulate etching performance of FeCl3. My experience with etching $\ce{Cu}$ circuit boards was that too strong solution was often etching metal under the protection layer. Be aware of proper after treatment, to prevent future corrosion.
Usual gloves, coat and eventually shield. Well vented place is needed, at least for first experiments, until you know the behaviour ( I mean rather passively - like outdoor - and just temporary for initial experiments. when it is settled, it is not much needed. ).