You don’t need per-device tokens in the firmware at all. Split it in two problems.
Wi-Fi: keep WiFiManager. Add one custom field in the portal if you want a friendly device name, otherwise use the MAC.
Token: let the node ask for its own. On the device profile open the Provisioning tab, set the strategy to Allow creating new devices, save, then Create key and copy the secret (it’s shown once). That key + secret is the same for all 12 builds. On first boot the node connects with username provision and an empty password, subscribes to v1/provision/res and publishes its request. The answer carries its access token, you store that in NVS and from then on it logs in normally.
// first boot only, after Wi-Fi is up
mqtt.setServer(host, 8883); // WiFiClientSecure, TLS
mqtt.connect(clientId, "provision", "");
mqtt.subscribe("v1/provision/res", 1);
String req = String("{"deviceName":"aq-") + mac6 +
"","provisionDeviceKey":"pk_xxx","provisionDeviceSecret":"ps_xxx"}";
mqtt.publish("v1/provision/req", req.c_str());
// in the callback: parse credentialsValue when status == "SUCCESS",
// prefs.putString("tok", value); then reconnect with the token as username
Two things bit us on a pharma site. New devices created this way land in quarantine, they connect but their data is dropped until an admin activates them, so tell whoever owns the org to look for them. And the name must be unique, a second request with the same name gets device_exists because it never hands out a credential twice. So never wipe NVS casually. Details are on Adding and claiming devices.