If you take extra funding and hire people just to meet their needs, you no longer have a SaaS. You are writing a custom product for this large customer, with explicit permission to try to sell it to other people too. But this big player will be driving everything, as if you do not do what they say, they can walk and your business collapses.

That does not necessarily mean it is the wrong choice - if you are not growing otherwise, and this is a path to keep it alive, it makes a ton of sense. But if you are growing and these guys are just a jumpstart to bigger ARR, it might be a mistake.

As for what I would do - I'd simply tell them that they are more than welcome to become a customer, and that you certainly will take their needs into consideration when prioritizing your roadmap.