Working in child theme is ok, as long as you know how to manage child theme. A child theme does not guarantee the original theme updates will work always with your changes, but at least you have them in one place, separate from the code.
I can guide you do css changes in child theme, but more complex changes require you study wordpress codex and know how to manage the code. Or you would have to update and add the changes back.
I am sure neither options sound great, but unfortunately these are the only possibilities.