Hi!
My school have a machine called a flow cytometer which basically uses lasers to identify the organelles in cells and the complexity of cells. It is used in medicine such as in blood testing applications, ecology as well as in other fields such as in genetics as you mentioned because it allows for scientists to identify multiple factors related to DNA and the cells(s) they are investigating.
Flow cytometry incorporates biology and physics perfectly, but of course it would require a decent amount of research if you do not already have experience with it, and I know you said you want to focus on a-levels which is completely understandable.
What do you want to do after a-levels, if you don't mind me asking?
Something related to biomedical engineering seems quite good in involving both, and even whether there is extraterrestrial life could be a good one considering there could be a decent biological aspect to an already very physics heavy topic.
Sorry if these ideas were not too helpful, but I wish you success with your EPQ!
For purely just biology there is so much, such as you could base your research on what the most effective way of genetic testing is or even on investigating a not very well known condition which is thought to be genetic etc
For physics, as you mentioned being interested in space, you could perhaps talk about the moon landing controversy, extraterrestrial life and its possibility or even something to with the extent to which other planets are habitable etc.
In terms of the difficulty, of course EPQ is a very time consuming process, but I would personally only do it if you know you can balance it as well as get the grade you want in it. I'm currently in year 13 and got an A in my EPQ last year, but as a dental applicant it did not really help me at all, just adds to my UCAS points to be honest, but I have friends who did it, got an A and got reduced offers for courses related to their EPQ such as pharmacy, so it's always worth considering if it is worth your time as well. I know someone who got a B in their EPQ but has an offer from the top russell group unis to study maths, as well as other great unis so do not let it stress you out too much. I would definitely recommend doing it if you want to do it though, its very beneficial in terms of the skills you learn- even though it did not help me personally, I am happy I did it.
Sorry this was not very helpful and very long, but I hope it will all work out for you!