A decent interface between fillers and the polymer matrix is a pre-requisite for optimal mechanical performance of polymer composites. Especially for extrusion-based additive manufacturing, an increasingly popular manufacturing technique for thermoplastics that is also known as 3D-printing, an optimised morphology is a must for complex composites, as the processability fully relies on morphological aspects. Filler agglomerations in the filament, for example, regularly result in clogged nozzles and buckling of the filament in the printing head. In turn, a prominent change in morphology can drastically alter the rheological behaviour of the composite. Consequently, the interfacial weld strength and, thus, the overall mechanical properties of 3D-printed parts can change. The present study determines the consequences of different compatibilisers, coatings, and glass sphere types on the properties of highly-filled PP composites that are most relevant for 3D-printing, namely morphological, rheological, tensile, thermal, impact, and dimensional properties. It is found that the most promising compound, comprising of 30 vol.-% coated borosilicate glass spheres and a compatibiliser based on maleic anhydride, reveals a homogeneous filler distribution and an exceptional filler-matrix interface. These findings combined with optimised processing settings that overcome the increase in viscosity offer an improved processability, dimensional accuracy, and mechanical properties compared to neat PP.