convert a 3D JSON object into a "fake" 3D associative array in Bash

I have object in JSON that has the structure as shown below and I want to convert it into a "fake 3D" associative BASH array with the following structure:

SHIPS[$mmsi:mmsi]=367513050
SHIPS[$mmsi:lat]=42.380329
...etc

In other words, I want the mmsi value combined with the key from each object to become the index to the associative array.

Right now, I am doing the following. The drawback is that it relies on the mmsi field to be the first field of each JSON object — which is probably true but also not very reliable.
This provides 3 objects: VESSEL_INDEX with all mmsis, KEY_INDEX with all existing keys, and VESSELS that can be accessed as VESSELS[$mmsi:$key]

EDIT: for clarity/completeness, my final goal is to:

• declare an associated array
• curl in a JSON file from a URL
• update the associated array (with keys VESSELS[$mmsi:$key]) with the keys/values provided in the JSON object while leaving any unmentioned elements of the array unchanged.

    declare -A VESSELS
    declare -a VESSEL_INDEX
    declare -a KEY_INDEX

    while read -r keyvalue
    do
        key="${keyvalue%%=*}"
        value="${keyvalue#*=}"
        if [[ "$key" == "mmsi" ]]
        then
            mmsi="$value"
            [[ -n "$mmsi" ]] && VESSEL_INDEX+=("$mmsi")
        fi
        [[ ! " ${KEY_INDEX[*]} " =~ " ${key} " ]] && KEY_INDEX+=("${key}")
        [[ -n "$mmsi" ]] && VESSELS["$mmsi:$key"]="$value"

    done <<< "$(curl -sL "$AIS_URL/ships_full.json" | jq -r ".ships[]|to_entries|map("(.key)=(.value|tostring)")|.[]")"

with this input curled in:

{
  "count": 2
  "ships": [
    {
      "mmsi": 367513050,
      "lat": 42.380329,
      "lon": -71.042946,
      "distance": 0.317654,
      "mmsi_type": 1,
      "level": -28.161266,
      "count": 6895,
      "ppm": 4.918982,
      "heading": null,
      "cog": null,
      "speed": 0,
      "to_bow": 10,
      "to_stern": 17,
      "to_starboard": 3,
      "to_port": 5,
      "shiptype": 52,
      "msg_type": 0,
      "country": "US",
      "status": 0,
      "callsign": "WDG2188",
      "shipname": "VINCENT D. TIBBETTS",
      "destination": "BOSTON",
      "last_signal": 0
    },
    {
      "mmsi": 367447520,
      "lat": 42.324032,
      "lon": -70.994347,
      "distance": 3.777312,
      "mmsi_type": 1,
      "level": -37.657475,
      "count": 1103,
      "ppm": -1.157407,
      "heading": 156,
      "cog": 155.5,
      "speed": 28.4,
      "to_bow": 24,
      "to_stern": 20,
      "to_starboard": 9,
      "to_port": 4,
      "shiptype": 40,
      "msg_type": 0,
      "country": "US",
      "status": 0,
      "callsign": "WDF4062",
      "shipname": "SALACIA",
      "destination": "XX XXX>?? ???",
      "last_signal": 0
    },
  ],
  "error": false
}