This study examines the reactivity and structural evolution of biochars obtained from apple pomace (AP) and chokeberry pomace (CP) during the Boudouard reaction. The raw materials were characterised in terms of elemental composition and fibre content, showing significant differences in the lignin and cellulose fractions. Biochars were produced by thermal treatment at high temperature and sieved to a fine particle size (<100 μm) for further analysis. The Boudouard reaction was investigated using thermogravimetric analysis under controlled heating and isothermal conditions at 750–850 °C. The results demonstrate a strong temperature dependence of the reaction rate, with significantly shorter reaction times at higher temperatures and noticeable differences between the two biochars. Additional laboratory-scale experiments were performed to identify optimal physical activation conditions, balancing carbon conversion and surface area development. Quantitatively, the reaction time at 850 °C was approximately 25 min for AP and 43 min for CP, while at 750 °C it increased to approximately 262 min and 480 min, respectively. Physical activation at 800 °C led to a significant increase in the specific surface area, reaching values on the order of ~1000 m2/g, depending on holding time, while maintaining high conversion levels. The results indicate that both temperature and residence time play critical roles in controlling the reactivity and textural properties of the obtained biochars. Structural analysis revealed that CP-derived biochars exhibit greater heterogeneity, whereas selected samples exhibit improved structural uniformity. Furthermore, potassium oxalate was applied as a mild activating agent, significantly enhancing the reactivity of the biochars by promoting the Boudouard reaction. The novelty of this work lies in linking biomass composition to Boudouard reaction reactivity and porosity development under both physical and mild chemical activation conditions. The findings provide a basis for tailoring the properties of biochar through controlled activation strategies and can be applied in the design of advanced carbon materials for gasification processes, CO2 utilisation, and adsorption applications.