Tyler’s right in one line, here’s the longer version. PubSubClient only reads from the socket and sends pings inside client.loop(). Its default keepalive is 15 s. You sleep 30 s, so no ping goes out, the broker drops you for keepalive_timeout, and state -3 is “connection lost”.

Never block in loop(), use a timer instead:

unsigned long last = 0;

void setup() {
  ...
  client.setKeepAlive(60);
  client.setBufferSize(512);
}

void loop() {
  if (!client.connected()) reconnect();   // with a back-off, not a tight loop
  client.loop();                          // every pass
  if (millis() - last >= 30000) {
    last = millis();
    snprintf(buf, sizeof buf, "{"temperature":%.2f}", bme.readTemperature());
    client.publish("v1/telemetry", buf);
  }
}

Keepalive between 30 and 300 s is what the docs recommend. And do what Niall said, the Log tab names the reason for every disconnect.

No per-device topics needed. All 300 publish to the same v1/telemetry. Hermetry knows who’s talking from the credential the device logged in with, so the token is effectively the address. Unit 17 can’t write as unit 18 because it doesn’t have 18’s credential, and a device can only subscribe to its own downlink topics (v1/attributes/set, v1/rpc/req/+ and so on), so it never sees another device’s messages either. No ACL file to keep.

Kevin’s not wrong that Custom topics exist, but they’re for firmware you can’t change: the profile then takes any topic outside v1/ as telemetry of that device. Limits there are topic depth 8, length 128 and 50 distinct topics per device, and with “Prefix keys with the topic” grid/s12/temp becomes a key like grid.s12.temp. Since you’re writing new firmware, skip all of that.

For the site id I’d make it an attribute of the device instead of part of the topic. It doesn’t change every minute so it doesn’t belong in telemetry.

Reference with the full topic table: MQTT topics and payload reference. One practical thing from when I moved my own stuff: give every unit its own credential. Two devices sharing one token keep kicking each other off.