What can I do if I want to upload my track but it is larger than 6MB? Other than making it smaller removing blocks. Any way to compress the file or something?

You can try identifying heavy custom blocks and get lighter versions of them on item.exchange. Usually, custom blocks you create within the game can be about 800 kb each. Browse here to find custom versions of the blocks you need.

If you used vanilla blocks only there's likely no way to reduce filesize sadly.

You can also support the ManiaExchange project and upload maps on here with sizes up to 24 mb. They will however likely not work on online servers (game limitation).