I'd take #QGIS Standalone 2.18or3 qgis.org; install plugins 1. SRTM Downloader and 2. QuickMapServices (or Vector Tiles Reader) then "Project > Save as image"!
Mit uMap kannst du in einer Minute Karten mit OpenStreetMap-Hintergrund erstellen und sie in deine eigene Internetseite einbinden.
Probably some postprocessing is needed which I would do with ImageMagick or IrfanView for GUI-based corrections. If more geodata-related bbox cropping or GeoTIFF conversions are needed I'd recommend #GDAL CLI tools (gdal_translate); see e.g. this intro