Are there any extension/add-on tools to improve report-writing in SSRS? My company is implementing some software which uses SSRS as the engine for report presentation both in and out of the application, so we'll be customizing a lot of SSRS reports soon.

Microsoft has put a good amount of functionality in SSRS but, frankly, Report Builder et al are still not the easiest tools to use for report-writing...

My Google searches seeking extensions or add-ons or other third-party tools to help with the report and query writing tasks doesn't find anything except what Microsoft has provided... Is this really the case, that there are no tools to improve on 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