How to migrate data from custom site to WordPress?

Currently I have a site with around 6.000 articles running on a custom built cms platform, but due to demand from the client, we have been tasked to convert all data to WordPress. Everything is stored in MongoDB in a custom structure (nothing extraordinary) and I have been looking at how I could convert the data to match the XML structure of WordPress for easy import, though not sure if I have to build such conversion script myself, or if an open source data-pairing tool exists, where I can match a MongoDB query and its data to the designated fields in a WordPress DB.

The article contents have been structured using EditorJS which acts a lot in the same way as the Gutenberg editor in WordPress, though I would need to know, how I should convert the data to match the Gutenberg editor, or if I should simply render the data as HTML and save it as such in the XML for import?

Categories are stored in a separate collection with a parent -< child structure, and pages pretty much acts as normal html.

Q: How would one go about converting this to WordPress data?