You should begin by summarizing your overall research question, and then describe your initial quantitative study and its results. Following that you, should describe the design for the follow-up qualitative study, with the additional realists that it produced.

This should all be relatively straight forward, so you maybe "over-thinking" this task.

There are various models for writing an abstract of a research paper or thesis; for example, the IMEaD Model, John Swalves Model (1996, 2004), Hyland Model (2000), and you can choose the one according to your research design. You can apply the IMRaD Model for a mixed-method study. It suggests briefly stating an introduction, method, result and discussion.

In a sequential explanatory mixed-methods study, the summary or abstract should clearly present the research process in a structured manner. It begins with a brief introduction to the research problem and objectives, highlighting the need for both quantitative and qualitative approaches. Next, it outlines the quantitative phase, including data collection methods (such as surveys or experiments) and key findings. Following this, the abstract describes the qualitative phase, explaining how interviews, focus groups, or observations were conducted to further interpret the quantitative results. The final part of the abstract summarizes the integration of both phases, emphasizing how qualitative insights help explain the numerical data. It concludes with the main findings, implications, and significance of the study. The abstract should be concise, coherent, and reflect the sequential nature of the research process.

given It is seq explanatory MMs, you should generally start writing a little bit of your study backgrounds and problems, followed by problem-solving solutions (e.g., what frameworks or methods (MMs) you use to tackle that problems related to your study backgrounds), then proceed to results and findings (e.g., summarise what seems prominent), and conclusions ( e.g., implications, suggestions, limitations). ps, just be extremely concise.