I am not going to
dive into this very far except to ask some questions whose answers will help later on.

If you are going to claim that NT baptism is the sign of the Ab. covenant, then you must be conversant on the Ab. Cov.

1) What is the Abrahamic covenant?
2) What was its sign and what did it signify?
3) Did the sign convey or apply any spiritual blessing, result, change or effect?
4) Was there any significance to the sign in the sense that it typified anything that was fulfilled in the NT?
5) Do you believe that there is anything significant about the mechanics of this covenant.
a) Should its sign always be something that can be given to your offspring, i.e. applied externally, as circumcision was?
b) Should the thing signified by the sign stay the same forever.
c) Or is it possible for it to signify one thing in the OT economy and something different later?
d) Same question for the sign itself.
e) If the sign conveyed or applied any spiritual blessing, result etc. , should its NT counterpart do exactly the same or can it change to a different applied result or no result at all?

That's quite a lot right there. The first one is the toughest by far.

Won't get to any answers tonight.

Lest you think I am the guy with answers, I am not. I just know that this is where the action is.