COVID-19: Individual level, cross-country Panel dataset. Which method to use?

I am struggling with the most appropriate method to use for my Master thesis on the impact on mental health indicators of informal caregivers before and after the start of the COVID-19 pandemic. I will use SHARE panel data for 3 countries and have also access to a special COVID-19 Survey from SHARE, asking three indicators of interest. My hypothesis follows the assumption that informal caregivers show a difference in the outcome indicators of mental health after the start of the COVID-19 pandemic. I will also analyze whether I see a difference between the three countries and translate it to their COVID-policy to support informal caregivers and long-term care system.
First, I got advised to use a Dif-in-Dif, but I have no control group, since everybody is affected by the pandemic. Then, I was thinking to use a fixed effects methods, but the special COVID-19 Survey does not ask the same variables that could change over time as the normal questionnaire does. It does include plenty of other variables concerning COVID, but these are questions asked for the first time in this Survey. I am not advanced with STATA, so could use all the help regarding the best method to use in this case.