I agree there is no "sure thing", but you still have grades on players that seperate them and establish value as prospects. We can call them whatever we want, but in the simplest of terms the top 5 grades in the draft are going to be more valuable than the 30th ranked player who teams have a 2nd round grade on.
As far as RB values I agree with you depending upon the rest of the plan. If you plan on keeping Pollard waiting on a RB is probably the thing to do. Looking at the last couple of drafts the 3rd round RBs were:
2022 - Rachaad White, Tyrion Davis-Price, and Brian Robinson
2021 - Trey Sermon
Sermon has been a bust so far, Rachaad White was a backup for the worst rushing team in the league, but flashed at times. Tyrion Davis-Price I bet 90% of fans couldn't guess who he plays for without looking it up. Robinson I like a lot and flashed at times, but was also in Zeke territory in YPC and offered minimal in the pass game.
If you move on from Pollard and need a plug and play starting running back rounds 1 & 2 are where you need to look. You're right you can get some great values later on, however it seems pretty rare that they can run, catch, and block well right away. Most mid to late round backs are tandem backs with one dimensional skillsets that need a year or two to develop.