Hi bghouse,
The first step would be to identify if your “symptoms” are from a mattress that is too soft and doesn’t provide enough pressure relief and contouring in which case a topper can help (see the topper guidelines in post #8 here) or if your “symptoms” are more alignment related from a mattress that is too soft in which case your options would be very limited because you can’t really make a mattress that is too soft any firmer without replacing the layers that are either too thick or too soft although there are a few options in post #4 here that may be helpful as a partial or temporary fix.
Post #2 here along with some of the posts it links to may give you some insights into what is happening and the underlying cause of your symptoms but based on that fact that your mattress is a luxury plush euro pillowtop it seems to me that your mattress is probably in the “too soft” range rather than the too firm range so a “fix” will be more difficult.
Phoenix