The migration path from Drupal 8 to Drupal 9 is not always smooth and easy. But there are also other good reasons to start with a fresh Drupal 9 website and to migrate the content from Drupal 8, as a migration is always a chance to improve important dimensions of a website such as the design or SEO (Search Engine Optimization). If you chose this path you can export/import the data as follows with CSV-Files and the help of some spreadsheet processing.
Export data from Drupal 8
1. Content Export CSV
Quick and easy to install and use, but does not allows for any customization. But is often enough as you can easily manipulate data in a spreadsheet with some related skills.
"This module is used to export your content of a specific content type as CSV file. You can find the content export page, under Configuration->Webservices->Content Export."
2. Views Data Export
The installation can be tricky if you lack the expertise, the configuration takes time, but you can really do anything you want to get the data exactly in the desired format.
"This module is designed to provide a way to export large amounts of data from views. It provides a display plugin that can rendered progressively in a batch. Style plugins are included that support exporting many types."
For the export CSV, it now has the option to keep the HTML (for example for the body field), in the Views, under Format (CSV File) Settings.
Import data into Drupal 9
This is the ultimate tool to import data. Very easy to install, takes some time for configuration, but the configuration offers you a nice interface and many options.
"Feeds is the module for importing or aggregating data into nodes, users, taxonomy terms and other content entities using a web interface without coding a migration. Data can be imported from various formats, such as CSV, JSON, XML and RSS feeds."