I've just figured out a way to use Soundizz to transfer MusicBee playlists to Spotify. You can't use the M3u format to do it because of the way Soundizz tries to read the song locations. Instead, follow these steps.
Step 1: Delete all exported playlist files in This PC>Music>MusicBee>Exported Playlists (or where ever you have your playlists set to export). This will make it easier to locate the correct playlist file once you correct your preferences.
Step 2: Open MusicBee and go to Edit>Preferences>Library. On this menu, there is a small portion at the bottom for exported playlist preferences. There is a small drop down tab next to your exported playlist file path. By default, I think It's set to M3U but you are going to want to change that to PLS. Apply and save your preferences!
Step 3: Close the preferences menu and close MusicBee. Then check your exported playlists folder to ensure that all of your playlists have been exported in the new format. If they have not re-open MusicBee and open the playlist you want to export. Then export it by going to File>Playlists>Export Playlist.
Step 4: Now, open up the Soundizz web app. Log in and connect to your Spotify account. If it is not already selected click the playlists tab in Soundizz. Now a button should appear in the top right corner of the web app that says "Import Playlist". Click It.
Step 5: The first thing that should appear upon pressing this is a window with three options. Opt for the first one which is to import using a file. Upon Clicking this button it should bring up another window. Navigate to your exported playlist folder and WAIT? WHAT? Your playlist files aren't there?!?! It's okay don't panic. In the window select that you would like to look for all files instead of "Custom Files". Your playlist should appear and you should select it.
Step 6: Confirm your tracklist. Name your playlist. Then select Spotify when it prompts you. Viola! It should begin transferring.
Note that Soundizz is not perfect and you should expect a song or two not to transfer. It will tell you which songs have not transferred to Spotify when it is finished so you can go back and manually add the remaining ones. Also do not panic if your playlists do not show up at first. It takes a minute for them to go through, just be patient.
Hope this helped!
EDIT: upon further review, I noticed that Spotify is god awful at trying to find weird niche indie artists so I only recommend using this if your playlist is full of generally "mainstream" music.