You're right that it isn't concrete, I agree, but I'd have to disagree with some of your reasoning.
Yes, Makima has never started with this move, apart from the one time against a Primal Devil. But this simply suggests that it's one of her more exclusive powerful abilities, which is why it was her opening move against the Darkness Devil. Now keep that in mind.
Makima wouldn't need this for the 20% Gun Devil. Its takedown was specifically coordinated, as I explained here:
The takedown was done in a way to efficiently get the Gun Devil into Aki's corpse as soon as possible, hence why she chose not to use her stronger abilities.
As for Pochita, it would be practically useless, considering he could just regenerate. In Part 1, we see him pull off feats such as
regenerating his entire body without the use of blood.
Makima was also effectively getting blitzed by Pochita on multiple occasions, he was simply too fast for her. So even if she wanted to use her stronger abilities, she couldn't. That's the thing about it. (Don't ask me why Part 1 Pochita is faster than the Darkness Devil, let Fujimoto write his story and first explain why the concept of chainsaws can erase things from existence.)
What determines whether or not Makima uses her ability is the opponent’s status, which in Gojo's case is pretty damn high. Mind you, the same thing happened with the Darkness Devil. Makima doesn't know what or how he works, but she's aware of his status as a Primal Devil or at least we're led to believe so. I believe the same reasoning would apply here with Gojo. It's the simplest ability in her kit to use for a potential victory and one she was actually shown to use against stronger opponents. I’d find it silly to think she wouldn’t utilize it against someone, considering she has genius intelligence and at least gifted battle intelligence.
As for Makima starting off with Bang, I'd honestly have thought the same thing, but now I kind of disagree. Why? The only times Makima has ever used Bang were against lower tier opponents like Power and her Blood Devil form or to BFR opponents. She’s never really used this ability as an opener against strong characters.
She was probably just testing the new ability she got after defeating the Gun Devil.
So mainly, when you place all the pieces together, subjectively for me, bio hax would be Makima’s go to.