As they increase enemy HP over time, I'm guessing that won't be possible in the future.
Increasing enemy HP is less important than the cycle allowance. It doesn't matter how hard they make the fight; full-clearing MoC 12 requires that you do both sides together in 10 cycles. Given that that's what you're building to do, you just don't need a sustain that can keep you alive long-term.
Sustains are an odd class because they're extremely important for letting you access higher tiers of content early on, but then they start falling off. The more damage you do, the less you care about sustaining, and the hardest content outside of SU is primarily a damage-per-cycle check rather than an all-around hard fight. "Don't use a sustain" is probably good advice for almost everyone trying to get more stars in Pure Fiction, and even in MoC you don't need a whole lot of protection given the time constraints.
My own experience is that Gepard was really useful early on when first going through the story, then struggled to keep up when I might need 7 cycles to finish a side in high-end MoC, and now is very comfortable again because I only need him to last 3 or 4 cycles. I no longer feel much pressure to upgrade to Fu Xuan or Aventurine.
