Running a shell script with ProxyCommand that computes an IP and starts an SSH session

I would like my SSH config to call out to a script in certain situations to resolve a different IP address than what I provide. In this situation, I don't actually want to proxy through another host, simply change the host that's being SSH'd to (although in some situations I would like to proxy through a host, hence bundling this into one script).

This is a rather long script and isn't really designed to be a oneliner in the ProxyCommand directive.

This is an extract from the script that shows the issue:

#!/bin/bash

instance_id="$1"
user="$2"
port="$3"

if [[ $instance_id == *"i"* ]]; then
  filter="--instance-ids ${instance_id}"
else
  filter="--filters Name=ip-address,Value=${instance_id}"
fi

public_ip=$(aws ec2 describe-instances $filter | jq -r '.Reservations[0].Instances[0].PublicIpAddress')

if [[ $public_ip == "null" ]]; then
  true
else
  ssh $user@$public_ip -p $port
fi

And this is my ~/.ssh/config:

Host i-* mi-* 172.2* 172.3*
   User cbell
   ProxyCommand ~/.ssh/aws_iea_proxy_command.sh %h %r %p

If I try to ssh to an instance ID, eg ssh i-030ee3dd4f2712b3b, the script is triggered, but I'm not given an interactive shell.

[connor@Enigma:~/.ssh] 130 $ ssh i-030ee3dd4f2712b3b
Pseudo-terminal will not be allocated because stdin is not a terminal.
-bash: line 1: SSH-2.0-OpenSSH_8.2p1: command not found

How do I get the SSH command running in a script via ProxyCommand to work?