Computational models (CMs) in cardiac electromechanical (EM) have shown to be an increasingly useful tool for investigation of cardiac function, general consideration of the influence of pathologies on it, potential treatment outcomes, and much more. Model personalization of any kind (image data, ECG-extracted data, EAM, etc.) has further upheld the role of CMs in, among others, aiding diagnostics, personalized drug therapy and presurgical planning. However, growing complexity requests additional care to ensure computational efficiency of CMs, their robustness for personalization and clinical condition variability. Surmounting the mentioned methodological challenges is therefore of utmost importance when having automated, scalable, high fidelity, physiologically complete and even clinical-grade models and workflows pinned as the final goal. The current contribution focuses on the investigation of the left atrial (LA) function in health and under fibrotic remodelling. The model consists of 3D LA, with geometry generated from the patient's image data (MRI, 13 patients), integrated with a physiologically comprehensive 0D CircAdapt model for closed-loop circulation. Further personalization is done by calibrating patient-specific early activation sites and conduction properties to match the ECG data, and through parameter adjustments based on both imaging and ECG data. Various experimentally examined scenarios, including alterations in loading conditions and contractility, have been considered for healthy models and have shown to replicate expected physiological behavior. Furthermore, 4 models with fibrotic remodelling, exhibiting different fibrotic burdens, have been considered. Fibrosis distributions obtained through late gadolinium enhancement have been replicated in geometrical LA models, seconded by electrophysiological and mechanical changes induced in diseased tissue by altering conduction velocity and L-type Ca current, and cellular contractility and tissue stiffness, respectively. Results, aiming to display the effect of fibrotic remodelling on the LA function, exhibit a reduction in generated active tension in diseased and healthy tissue, with varying magnitude, prompting reduced atrial wall motion. The current study showcases a multifacetedly personalized, high-fidelity, physiologically complete application of a CM for cardiac EM parameter space explorations over prolonged periods the prediction of fibrotic remodelling on LA function.