Moreover, most roadmaps I've been a part of have all involved items where the first step is discovery, both product and technical. There's usually strong signal that it's the right thing to do, but the first step isn't writing code, it's figuring the thing out from different angles.

Exactly this.

The roadmap is not for the engineers to know what they are going to code next.

That comes through discussion and sprint planning in a tool that has tickets/milestones/epics etc.