Arcane flux can only proc once per round on a Spellsword it’s tied Elemental strike (instant attack). Wizards can cast cantrips twice per round. There is also mention of an internal cooldown which I haven’t tested around.
I respect the choice to make an investement to open chest in dungeons. I played a rogue (which has changed a lot since then but I am not talking here about the power level of rogue in the current version) before and just after the grenades where introduced and the fact that everyone with a familiar could do my job (when it comes to looting) was underwhelming.
As said before there are still many ways to open chest on a non full rogue - dips, gear swap, henchmen (tied to a different skill), etc.
As someone that has played and recycled multiple characters I appreciate the limitation every new build brings and enjoy as it makes playing Arelith feel fresh. I do admit that when I was a newer player I also tried to be able to access as much as possible (not saying that people advocating for open lock access on familiars are newbies just wanted to refer to my personal change to seeing limitations of game mechanics on different characters).