To change the expand/collapse icons of the TreeView I would also recommend using custom CSS.
The solution you have implemented is correct. I just want to add an important note. These styles will work only if the TreeView uses Font icons. This was by default up to UI for Blazor 4.3.0. After that, the default icon type was changed to SVG and you will need different styling for that. Here is an example: https://blazorrepl.telerik.com/GoYUmKbv15GO0B3v44.