I worked in and ran a digital agency for two decades. I have written the proposals, sat on the other side of the pitch table, and made the arguments agencies make. I now work independently, with no reseller agreements, no platform commissions and nothing to sell you at the end of a build.
That is the whole basis of this guide, and I will say it once rather than keep reminding you. Nothing here is an attack on agencies. Most of the ones I know are competent and are trying to do good work. The problem is structural. A supplier cannot be expected to scrutinise their own output, and no amount of good faith changes that. If your only source of advice about the work is the firm doing the work, you have no independent read on whether it is any good.
You are not stupid about this. You are unfamiliar with it, which is a different thing, and most writing on the subject fails to make the distinction. So I have written it the way I would explain it across a table.