For performance reasons it might not be the best idea to increase the size of the settings value in the database, as this would increase the size of all settings, not just for the custom CSS.
My recommendation would be to switch to external less files, which you can load from extend.php using a local extender. Those less files could be updated manually through SSH/FTP, or through a continuous deployment from a git repository or similar. The code that you might need to quickly edit can stay in the custom CSS setting of Flarum. This would also allow you to more quickly find the code you need to edit, rather than having to browser through one enormous file.
For an extension idea, something that would probably be most useful is the ability to create multiple custom CSS configs, with a name and a toggle. This would serve multiple purposes, make management easier and increase the CSS length in a way that can be managed efficiently. In fact, such an extension kind of exists, Scratchpad, but it would be better to create a fork and remove the javascript and PHP features to reduce the risk of getting your server hacked through the extension. Changes would also be required to make it run without debug mode.
