I guess you could say I main a DA on reboot, and I agree it needs work. I remember maxing my frenzy node and getting excited that surviving with it on was much easier, but bursting like you mentioned you lose all your fd. I haven't really bossed endgame content so I cannot speak to that.
I feel like you would need to do more than just a flat 10% dmg on frenzy, but thats a start.
As to how HP scales, I dont know how to balance that, I just don't understand why Innate HP on gear isnt normalized in proportion to the other main stats.
Its missing on some gear and HP value is higher than main stats on other peices.
Also Starforcing certain pieces gives you no bonus to hp, just str our secondary stat.
Problem is DA isn't weak at the start, just late to endgame it really falls off.