My perspective here is whether you are starting at a larger organization with hundreds of employees or a startup with 40 employees - the first 30 days look roughly the same. This is your window to build up as much context as possible, understand the goals, mission and prioritiies of the business, and identify gaps and areas for impact. The advantage of being at a smaller company is this access to information is much more accessible. People are easier to reach, things are probably still very evergreen - which means a lot of blank space for you to think about how you'd like to build things vs negotiating to adapt what others have built. So I'd spend those first 30 days really getting to know those 40 people - envisioning what you'd like marketing and pmm to look like in the long term and deciding what you can do today that will get you to that longer term vision - ie. start building those bi-lateral product/engineering relationships now.
Within the first quarter at a scaling B2B SaaS business, I'd imagine you'd want to take a look at how your story is showing up in the market - for your core audience. Do you know who your core audience is? Who is your ideal customer profile, what is their role, industry, company size etc. This is the time that you want to do the work to define this audience, and develop your positioning and messaging so as you scale you know where and how to reach these prospects in a way that converts. Once you have solid positioning and messaging it should be the foundation that your website it built off of, and your sales narratives are crafted from, etc.
Outside of core positioning and messaging, I'd take some time in the first quarter to define your GTM process, specifically for how to take new products and features to market. If you are starting to scale, and fast - you'll want a repeatable process in place that all of your cross-functional counterparts are aligned with - so you can spend less time re-creating the wheel each launch.
And lastly (although this list could probably go on and on), another advantage of being at a smaller and earlier stage company is access to customers. Carve out time every week to talk to customers, talk to prospects, listen in on calls - and start to build relationship with these early customers. Those relationships can often times lead to co-marketing, where customers are open to sharing your story through testimonials, customer stories, and positive reviews. These endorsements are crucial for early stage businesses.
Your first 30 days - 90 days will offer many areas to tackle that will have immediate and often big impact, your key role will be in defining how to prioritize.