I'd go on about this in an informal manner. I'd probably ask general open questions like "What did you like about the course?", "What do you think could be improved about the course?", and then cover issues I am particularly interested in, like "What do you think of the way the exercise feedback was given?", "How do you think students could be stimulated better to participate actively in the online lessens?" or "how effective do you think was XXX that I tried to stimulate participation?" etc.
I do not think that questions need to be unbiased. In fact I'd like questions to encourage students to be positive and constructive (therefore I ask "what did you like" but not "what didn't you like"). Many feel more comfortable writing positive things and some will respond to "What do you think could be improved about the course?" rather than "what didn't you like?" Also I think it will help me more and make myself more open to take criticism if it is presented in a constructive way, and I will more happily use ideas to improve if I get a generally positive message from the feedback. The students need to be encouraged to write things that can help you to improve, but I don't think it serves anybody to invite them to complain.
Obviously it is laudable that you'd also like to cover some aspects that are maybe not the first things you think of yourself. Some general aspects that can be covered are speed of delivery, quality of the provided material, do the students feel comfortable to ask questions and that your answers address them properly (the problem here is that students who for some reason don't feel comfortable to ask you questions may also not be comfortable to tell you this), what do the students think about the amount of work to be done, how this is organised, is the material challenging enough or even too challenging, do they think that you assessed their background knowledge well, i.e., neither spent too much time to explain things they already knew nor assumed material that they didn't know etc. Others will add more. You can of course also ask things like "Is there any comment you have that isn't covered by the questions up to now?" or "Do you believe that there is something about the situation of the students in these times that I should be aware of/take into account more?"
A word of caution though, somewhat in contrast to the number of suggestions before. Keep it short! Keep in mind filling in a questionnaire and then reading it takes time of your students and you. Time is precious. It is probably a bigger problem to cover all you can think of in a questionnaire that takes half an hour to fill in than to ask just five or seven questions and then realise that you forgot something you'd have liked to ask. With very open general questions, you give the students at least the chance to tell you whatever is not covered by other questions.
