What's the easiest way of importing content of a custom D6 content type into a D8 site?
The two options here is the inline D6>D8 migration, or to use the Migrate API + Migrate Plus (if needed) directly to write your own migrations. The latter has example migrations you can look at. There is never going to be a case or scenario where you will want to write a manual SQL migration script. You should not need Views or Feeds for this - the Migrate API has source plugins that can connect to databases or other data sources to query and pull data.

Here is a docs page outlining how to migrate from a SQL source:

https://www.drupal.org/docs/8/api/migrate-api/migrate-source-plugins/migrating-data-from-a-sql-source

You would set the two databases side by side in MySQL, create a connection setting as mentioned and create a source. The class controls certain aspects of querying and the migration YAML(s) map the fields to their new destinations.

For all the nuanced aspects, I would recommend joining the migration channel in the Drupal Slack, people are always willing to help there.