You can't delay upgrade. As soon as you reach ATP minimums you will be sent to upgrade training and ATP-CTP. You will be required to bid on the next vacancy bid. I believe per the contract you may temp out of your current assigned base until that happens.
Per diem is $40.00 a day so that is a plus. However, besides the two system bids in October and May you just won't know until it comes. STL could be an option or it could be all St Thomas and San Juan.
The pay is decent, especially as a captain and it is nice being under a contract with pay guarantee's per week. You will fly a lot but unfortunately basing is very transient until you have seniority to know what you can hold.
The good news is Cape Air embraces they are a stepping stone and people are constantly leaving the company. If you want anything besides the Northeast it should be easier to obtain.