STEM education has become a vital global strategy for nurturing 21st-century skills among learners. However, secondary school teachers in practical contexts still face significant challenges and lack the competency to design and implement integrated activities due to fragmented professional training in the past. This study aimed to assess the current and expected states of STEM education instruction among secondary school teachers, and to analyze and prioritize their professional development needs to enhance such instruction. Utilizing a mixed-methods research design, data were collected from a stratified random sample of 143 science and mathematics teachers in Savannakhet and Khammouane provinces. The research instruments included a 5-point Likert scale questionnaire, in-depth interviews, and focus group discussion guidelines. Quantitative data were analyzed using mean, standard deviation, and the Modified Priority Needs Index ($PNI_{modified}$), while qualitative data were examined through content analysis. The findings revealed that the expected state of STEM instruction was overall at a high level. However, in the current state, teachers were able to perform at a high level in only one dimension: connecting lessons to real life and careers. The remaining five dimensions were performed at a moderate level, with the creation of STEM instructional media from local materials receiving the lowest implementation score. Crucially, the $PNI_{modified}$ ranking identified the top three urgent teacher professional development priorities as follows: 1) using simulations or virtual laboratories ($PNI_{modified}=0.108$), 2) creating STEM media from local materials ($PNI_{modified}=0.100$), and 3) designing integrated STEM lesson plans ($PNI_{modified}=0.098$). These insights demonstrate to the academic community that sustainable STEM teacher development in developing contexts must focus on mitigating equipment shortages through digital tools and low-cost local materials, driven continuously by school-based Professional Learning Communities (PLC).