Are there any extension/add-on tools to improve report-writing in SSRS?
SQL Server Data Tools is typically used to author SSRS reports when Report Builder is insufficient. You can download and install SSDT as a standalone tool (runs a cut down version of Visual Studio) or if you have VS installed already it installs as an extension.

Make sure you download the correct SSDT version for the version of Visual Studio you have, then you can create and deploy advanced SSRS reports/projects from Visual Studio.

SQL Server Data Tools

Older versions for previous VS releases