How to write about subject of thesis if it is not completely known?

I am (tentatively) half way through my PhD in theoretical physics.

I am applying for a summer school, which asks me to write the subject of my thesis (if known). In my field, a thesis is often made of several research projects on different (but usually loosely connected) topics.

I have worked on several topics, and know about my upcoming research projects, but all the topics of my thesis are of course not known.

Can I write something like "Subject of thesis: topic 1, topic 2,... topic n?"

I am afraid that it may indicate there will be no other topic in my thesis, which is not true as I will work on future projects. How to acknowledge that?

I think you are overthinking this, what is expected in general is that you write the thesis topic(s) that you have currently, in a summer school it is just for organizers to be aware of what your (current) research topics are, to make decisions on acceptance.

There is no need to predict the future if not known. PhD research topics vary greatly with stage and overall project, so provide the information you have now and it should be fine.