is there some way to avoid things like 3+-5 appearing in questions
when I ask e.g. 3+{x}
and {x} is allowed to be <0?
I can't think of a way, but perhaps there is someone here more devious than me who can.
You could try STACK https://moodle.org/plugins/view.php?plugin=qtype_stack, but that is overkill for this simple case.
If x is always less than 0, then a set of parentheses around it might help read better. Since negation and subtraction use the same symbol, I doubt that you could indicate it with CSS.
If x is sometimes less than 0, then possibly something like: What is 3+{={x}<0?({x}):{x}}?