It's actually NOT a huge volume of work. There should be some level of discovery and market understanding happening in an ongoing manner, but IF a company is entering a brand new market, then yes, they should do research/discovery to understand what they are getting into. This can only help make much BETTER decisions and accelerate time to success/revenue etc.

PMs should be ahead of the delivery team. Yes. They have to have context for what they are looking to achieve but the delivery team should be included in the user research as well. i.e. they do it together,, I've done that several times. i.e. we had market understanding and high level signals that we wanted to focus in a certain area - e.g. workflow automation, for a product, but to get into the details we absolutely included the Eng team etc. as they needed to hear those details first hand and be part of that process.

IMHO, this process is simply good business management. The challenge I've seen is that people split product and GTM orgs and treat them as fully separate entities as opposed to finding an integrated view knowing that product leads to marketing, marketing leads to sales etc.