Other marketing roles have clear KPIs; PMM is the only role where we have multiple KPIs. Is there one or two that is critical for PMMs?

We have multiple KPI's and I'd argue that the bigger challenge is they are often shared metrics. Shared with our marketing, sales and product counterparts - which makes attribution often very very hard.

That said, as mentioned in earlier questions / answers - focus on the KPIs that you have the most influence over as a team. In my past, personally, we've focused a lot on product adoption - especially as it pertains to new features and releases as PMM was the driver in building out our GTM plan. Of course adoption also depends on the in-product experience of a user and how easy a feature is to find and use, but with PMM support in this area we can connect how a user engages with our marketing tactics (email, in product messaging and tours, content, etc) and how they then adopted the product / feature.

This also extends to new customer acquisition - specifically via new product and feature releases. You have likely built a plan around how to attract new customers with a new product or feature - and although there can be marketing spend attributed to these releases that contributes to the acquisition - as the PMM you are likely driving the GTM plan and how / where / when to attract these new customers.

I am a huge proponent of shared KPIs. Like Product Managers, Product Marketers often have to work through and influence other teams in order to reach their goals -- and, more aspirationally, to help their customers reach their goals.

Shared metrics have superpowers. They drive alignment, accountability, and accuracy overall. They're quantitative and qualitative. Bit of a broken record here, but I really like Qualified Pipeline Generation ($), Average Deal Size, and Meetings Set/Opportunity creation.

Other great shared metrics can be Trials Created/Converted, Win Rates, New Customer/Segment acquisition. New Product Introduction is a great time to dig in on shared metrics, too. Not just pipeline and influence, but Attach Rates (% or Count of current customers adopting the new product or service.

The counter example to this is the MQL. This is my least favorite marketing metric of all time. It's 100% subjective (what magic score means "qualified?". It's 100% internal (owned, managed, and measured my marketing, usually inside a marketing specific tool like Marketo). And it changes over time.