I like these goals! I'm a player who likes to have interesting choices to make, on build, equipment, and playstyle, so "a broad set of combat strategies in order to enable a wide range of playstyles" is something I value a lot.
This section seems very ... observational, and that worries me. I tend to expect that game designers have opinions about what roles their game should offer, and to try to shape the game so that those roles emerge. Instead, it looks like you're simply echoing back what the community has decided, and our "decisions" are just responses to the game's selection pressures. Was it your intent that these would be the roles?
What attracts me to this game, and distinguishes it from other MMOs I'm aware of, is the apparent flexibility and nuance to the choices players are invited to make. Looking at the skill and trait systems, it seems natural for players to come up with builds that can't readily be described with a simple, generic label like "healer". Every class has a specialization that rewards hybrid builds. Every trait line offers synergies that diverge from its core purpose. That's really cool, and I've always interpreted it as a thoughtful and intentional aspect of your design.
I appreciate you mentioning that "not every build needs to perfectly fit into a role". I hope that remains true as you approach your ideal form of balance.
How strict is this "purity of purpose" principle? There are lots of skills, traits, and weapons that fit two or three of these labels. I think this is essential to the game's nuance, and I think it supports your goal of having a diverse set of good choices available.
The combo system is particularly "impure". Depending on the combo field, a blast finisher could be boon support, soft control, or healing. Depending on the finisher, a fire field could be damage, boon support, or both. How does purity of purpose apply to skills with combo fields or combo finishers?
My hope is that this principle is aimed at each choice in isolation, and won't prevent interesting synergies that expand a choice's purpose beyond its core identity. My fear is that this principle will rob the game of its identity; it seems like a threat to the trait system, the combo system, and the elementalist profession, all of which I adore.