Okay, first please be sure that you’ve cleared your cache so that you’re loading the latest CSS, which supports styling for Font Awesome’s SVG with JS functionality. I’m assuming that’s what you’re referring to?
If you mean you’ve just removed all Font Awesome resources (no font styles or JS), and are adding SVG markup in manually, then naturally there wouldn’t be any icon present to display there and nothing would appear. In that case, there isn’t an automatic way to swap out SVGs, as the plugin expects the Font Awesome assets to be present.
If that’s the case, then your options would be:
1. Re-enable Bellows’ Font Awesome assets (this is enabled by default with the plugin, and will allow the icons to be displayed)
2. Style the submenu toggles using CSS. You might set the content to +/- for example (rather than using icons), or you could consider using the background-image property with url() setting the SVG either as a direct reference to the file or via a data string
3. You could use Javascript to dynamically replace the i tags with your SVG markup.
4. You could try using the walker_nav_menu_start_el filter to replace the i tag markup with your SVG HTML on the server side. Just be careful as this filter acts on an HTML string, so if this is done wrong it could easily break the menu markup.
Hope that helps