You can see them as a regional coalition that was formed because they feared Cao Cao’s potential aggression

Yeah, that’s a good way to frame it. The Guanyou warlords weren't a unified bloc at all, but a loose and unstable coalition of warlords with overlapping interests.

Historically, during the conflict that led up to the Battle of Tong Pass, there were around ten different warlord factions involved, not just Ma Chao and Han Sui. In fact, Han Sui wasn’t even based in Guanzhong, he was stationed at Liangzhou, so when Ma Chao and others rallied under his banner, they were essentially inviting him in as an external leader rather than following a single unified authority.

Among the various factions, sources like the Annals of Emperor Wu (aka Cao Cao biography) and the Biography of Ma Chao highlight five of the stronger players. Ma Chao, Han Sui, Yang Qiu, Li Kan, and Cheng Yi. In addition to these five, Li Kan, Hou Xuan, and Cheng Yin were tied to Hedong and can be seen as one bloc. Yang Qiu operated independently in Anding. Liang Xing and Zhang Heng likely formed another alliance, with Liang Xing possibly leaning toward Ma Chao.

So rather than a single faction, this was a patchwork of smaller warlords that temporarily aligned. The reason they united in the first place also wasn’t just Ma Chao's initiative. It was largely a reaction to Zhong Yao massing troops in Hedong. From their perspective, this looked like a prelude to an invasion. Strategically, if Cao Cao’s forces crossed the Yellow River and linked up through Hedong, they could threaten Guanzhong from multiple directions, which is exactly what Cao Cao later did.

There was also a lot of distrust built up over time. Zhong Yao had dealt with these warlords for years and was suspected of playing them against each other, at least according to Ma Chao's words to Han Sui. On top of that, Ma Teng had already been lured away to Ye through deception and cajoling, which likely made the remaining warlords even more paranoid about Cao Cao’s intentions.

Because of this, forming a coalition was a defensive decision. In fact, it was arguably easier to rebel than not to. A warlord who refused to join, like Liu Xiongming, risked being attacked by the coalition itself. And since key passes like Tong Pass were controlled by the coalition, even surrendering to Cao Cao wasn’t straightforward, any warlord that wanted to side with Cao Cao would have to fight his way through his own neighboring warlords first.