Additive manufacturing (AM), specifically fused deposition modelling (FDM), has emerged as a crucial fabrication method for polymer components because of its design versatility, material efficiency, and appropriateness for quick prototype and functional part production. Polyethylene terephthalate glycol (PETG) is extensively utilised in FDM applications due to its advantageous thermal stability, dimensional precision, and mechanical properties. Nonetheless, the surface quality and mechanical properties of FDM-printed PETG components are significantly influenced by processing conditions, constraining their wider use in load-bearing and functional applications. This work seeks to systematically optimise critical FDM printing parameters to enhance surface finish and tensile strength in PETG components. A systematic experimental approach was employed to determine process–property correlations. Two experimental phases were executed. The initial phase examined the effects of layer height, nozzle temperature, and bed temperature on surface roughness. The second phase investigated the relationship between surface features and mechanical performance by assessing the tensile properties of PETG specimens printed with various infill patterns, specifically rectilinear, concentric, and octagram spiral, as well as differing construction orientations. A Design of Experiments (DOE) methodology utilising Response Surface Methodology (RSM) was implemented to create predictive models, with the significance of parameters and model adequacy assessed through Analysis of Variance (ANOVA). The investigation confirmed strong anisotropy where the layer height significantly affected roughness at 90° (p=0.03388), but no significant model was found for 0° within the tested ranges. Surface roughness (Ra) ranged from 1.76 µm (minimum) to 5.81 µm (maximum) at 0° and from 6.29 µm to 11.70 µm at 90°. Mechanical testing demonstrated that tensile strength is significantly affected by infill pattern and build orientation. The concentric infill design demonstrated superior tensile strength, approximately 67% higher than the octagram spiral, due to improved load distribution and inter-filament continuity, whereas the rectilinear pattern provided an advantageous equilibrium among mechanical performance, material efficiency, and decreased printing duration. In all configurations, specimens produced with a 90° construction orientation exhibited enhanced tensile performance. This research presents a multi-parameter optimization and prediction framework for PETG FDM printing, facilitating tailored parameter selection to achieve an equilibrium among surface quality, mechanical performance, and production efficiency. The results validate the high-performance PETG components for functional-industrial AM applications.