Looking at the current source code, it seems like only "Original API" mode is supported for Mixpanel ID management.

Simplified ID Merge docs: https://docs.mixpanel.com/docs/tracking-methods/id-management/migrating-to-simplified-id-merge-system

Essentially, it just uses the $device_id and $used_id properties in track events to identify users (no separate identify calls needed).

If a Mixplanel project is created using a "Simplified API" id merge mode (which can't be changed), and we pass $device_id, and $user_id, I think it's being replaced in the following line:

Because ...getRequestBodyProperties(event, token) returns a $device_id property that has been set to a random UUID managed by this MC tool (in its own cookie), any $device_id we pass in the customFields is being overridden.

What do you think about switching the places so that any customField could replace the built-in ones, like this?

 properties: {
   ...getRequestBodyProperties(event, token),
   ...customFields,
 }, 

This way the $device_id and $user_id fields added to the tool settings (example: in Clodflare Zaraz dasboard) will have priority, and that way we can support "Simplified API" mode easily too?

Generally, I think customFields overriding any built-in properties the tool sets is a good and more intuitive idea.

Please let me know if I missed something. If this change is the only thing needed I can create a PR from a fork for it.