As of 21.07.2025, there exists a Bulk Data Ingest API for SFMC that is meant for bulk data import jobs.

You create a job definition, then upload data in "chunks", and close the job to initiate its processing. Afterwards, you can check the status of processing.

Uploading data into the job is called staging data. Data needs to be sent in JSON format. You are limited to 1000 data stage calls per job. Recommended size for a staging payload is between 2 and 4 MB, with a hard limit of 6 MB. So you are limited to 1000 * 6 MB JSON data in one job.

You can find the reference here: https://developer.salesforce.com/docs/marketing/marketing-cloud/references/mc_rest_bulk_ingest?meta=Summary