You can use Mastodon's advanced web interface to have as many columns as you want:
https://fedi.tips/how-to-activate-mastodons-tweetdeck-style-advanced-web-interface/
You can pin lists open in their own separate columns.
You can't add people you don't follow to lists, but you can hide lists posts from your Home timeline:
https://fedi.tips/how-to-use-the-lists-feature-on-mastodon/
(See the instructions at the bottom.)