Reliability and validity are both quantitative concepts that apply to surveys rather than semi-structured interviews. I recommend considering you first one or two interviews as pre-tests, and then adjusting your questions as needed.
A semi-structured interview is a qualitative method of data collection. It usually starts with a series of standardized questions leading to an organized conversation, which is guided by new information learned as the interactive discussion develops. For the credibility of its findings, you could follow an established framework for conducting semi-structured interviews, thus enhancing the objectivity and trustworthiness of your study. To that end, you might check out the insights proposed by Adeoye-Olatunde and Olenik (2021) and Kallio et al. (2016), fully cited below.
Adeoye-Olatunde, O. A., & Olenik, N. L. (2021). Research and scholarly methods: Semi-structured interviews. JACCP: Journal of the American College of Clinical Pharmacy, 4(10), 1358–1367. https://doi.org/10.1002/jac5.1441
Kallio, H., Pietilä, A.-M., Johnson, M., & Kangasniemi, M. (2016). Systematic methodological review: Developing a framework for a qualitative semi-structured interview guide. Journal of Advanced Nursing, 72(12), 2954–2965. https://doi.org/10.1111/jan.13031