Sending Assisted GNSS (A-GNSS) to Bangle 2 GPS (AT6558)

The only my concern is, they need to be pushed one by one, after ACK for each message is received.

I assume you have sent these one by one manually. You probably only need a 30ms gap between each one. You can chain timeout or use promises. Or a really crude way.

send1() {
      Serial1()....
      setTimeout(  send2(), 25); 
}

send2() {
       Serial1()....
       setTimeout(  send3(), 25); 
}

On Bangle 1 - we just sent the data with enough delay and did not check for an ACK.
It would be possible to measure the time it takes to get the ACK back.