Problem is most top starters had a bad year or two starting their career, Nola is an exception, go back and look at Halladay's first seasons:
22: 149 IP, 3.96 ERA, 5.36 FIP
23: 67 IP, 10.64 ERA, 6.47 FIP
24: 105 IP, 3.16 ERA, 2.23 FIP
25: 239 IP, 2.93 ERA, 2.97 FIP
The point is simply that with young pitchers, you have to focus less on performance and more on projection.
To me, Eflin has the highest upside because he has velo, 94-96, and a full repertoire of pitches, he needs to improve his secondary offerings and his command, but we're talking tweaks.
Pivetta belongs in the pen because he's a two pitch pitcher at 26, has a velo uptick in the pen, but shows no signs of either developing movement on his 4 seamer or developing a 3rd pitch.
Velasquez is on the bubble to me, he has the stuff to start but his command problems drive up his pitch count, He'll probably end up in the pen as a two pitch (FB/slider) RP because he hasn't been able to master the changeup.
DLS is headed to the pen if he can't develop that third pitch, that seems to be the back breaker for potential starters these day, having three pitches you can command at different speed and movement to keep hitters offbalance.