Castable alumina forming austenitic (AFA) alloys have demonstrated superior creep life and oxidation resistance at temperatures exceeding 800⁰C. Despite the success in applicability of these alloy in extreme environments, there is a limited understanding of the deformation modes and the influence of each alloying addition guiding the alloy design strategies that could further enhance the creep strength of these AFA alloys, particularly at temperatures at and above 900⁰C. In this study, we reveal the mechanism underpinning the superior creep performance of AFA alloys that involves suppressing primary carbide formation through minor compositional modification in castable AFA alloys. This approach results in a three-fold increase in creep strength at 900⁰C and 50 MPa. By employing integrated characterization techniques, we analyzed the microstructures of two AFA alloys both before and after the creep process. We discovered that the suppression of primary carbides permits the in-situ clustering of now-available interstitial elements such as C, Si, and O during high-temperature creep. This improves effects related to solute solutionization such as solid solution strengthening and a reduction in stacking fault energy. Moreover, also enabling controlled secondary carbide formation during creep, and where retained tungsten in the solution further provided strengthening. These findings underline the important interplay between alloy composition, microstructure, and creep properties, and offer a promising design strategy for developing economical high-temperature Fe-based alloys suitable for advanced applications.