Filter Klaviyo data via PHP or / and JavaScript

Hello Klaviyo team.

In addition to the following ticket (https://ww.wp.xz.cn/support/topic/how-can-we-add-additional-data-to-a-tracked-order/), I would like to add that it would actually not only be good to extend or modify order data before sending it to Klaviyo, but actually all data/events that the Klaviyo plugin sends.

Then, with the help of a simple WordPress developer, any store operator can easily extend and customize data objects that are sent to Klaviyo with relatively little effort.

Backend filters in PHP are already available in WordPress as a tried-and-tested solution.
Frontend filters could be made possible via JavaScript. WordPress offers these for developers in Gutenberg, for example, so you can take inspiration from WordPress when implementing JavaScript filters.

We are generally not really satisfied with the current way that we are referred to the docs to make simple, minor adjustments.
The Klaviyo API docs seem way too complicated to me.
It takes way too much time to understand it, then develop and test if you just want to customize or extend a small data point.

It would be much easier if I could just modify or extend the data objects that Klaviyo sends from WordPress to itself anyway.
Put my data into simple PHP arrays or JavaScript objects and Klaviyo will convert the filtered, simple data objects into the API structure it needs and just send the HTTP request as before.

Examples:

• I would like to give a user profile a language code as a property. Ideally, this should always happen when Klaviyo creates or edits the user. (Probably not possible at the moment.)
• I would like to add some metadata to a WooCommerce order entered in Klaviyo (or a checkout abandonment) before the data is sent. (Currently not easily possible.)
• Other support tickets have asked for similar requests in the past. e.g.: https://ww.wp.xz.cn/support/topic/filter-request-5/ or https://ww.wp.xz.cn/support/topic/additional-order-lines-in-events/