I don't think any of the examples named are really good examples of what Fields would be trying to do on a 2nd team.
Favre- 1 year, 0 starts for Atlanta
Harbaugh- was a 7 year vet with 65 starts by the time he went to Indianapolis
Steve Young- had 19 starts in Tampa, but then sat for 4 years in SF
Orton- had 33 starts as a Bear, he wasn't very good, but did win 21 of those starts. He lost 21 of his 33 Denver starts
Flutie- only 1 Bears start, 13 in a couple years in NE, then a 9 year break between starts in his prime
Smith- 75 starts in SF, 76 in KC.
I thought of guys like Hasselbeck (didn't flame out in GB), Geno Smith (sat on bench, then broke out 8 years later), and Sam Bradford (never really all that good)
Brees is an interesting comparison, but he turned the corner in Year 4 still in San Diego. And the shoulder was a big reason he was let go. The equivalent would be the Bears taking Caleb (then trading him to the team that takes Drake Maye), sitting the rookie, while keeping Fields next year when he finally breaks out.