So @pickles, I can offer some first-hand advice here.
My parents and other family members refused to attend my outdoor, secular wedding. This was 30 years ago. Same situation as yours. I was raised Catholic but was agnostic.
So here is what I can offer you. Don’t try to convince or change your parents’ minds. The best you can do is let them know what you believe (couldn’t tell from your OP if they know you are agnostic). Honesty is important, because they deserve to know the truth and you shouldn’t hide the truth of who you are from others. Then remind them you and your fiance are people of integrity and simply aren’t able to participate in a wedding at which you would be required to make promises you have no intention of keeping. You may go so far as to explain your moral objections to what a Catholic marriage entails. That is up to you.
As a previous poster said, disappointment can be mitigated by adjusting your expectations. Your conservative Catholic family members most likely aren’t going to come to your wedding. You need to plan ahead and fill those seats with friends and family who do want to come and celebrate your marriage with you and your fiance.
Consider this a lesson of forgiveness. Most will say to focus on accepting your parents and their beliefs, just as you wish them to accept yours. I don’t know how productive this would be. A child deserves to be able to expect her parents will be there on the most important days in life, to the extent they are able. Birthdays, graduations, weddings, birth of children, etc. When a parent falls short because of their religious beliefs, it is a hard pill to swallow. Somehow I was able to put it behind me with the attitude of “What is done, is done” and remembering that there are times I am sure I hurt people in my life by just being who I am. We all need to be authentic.
Plan your wedding. Shop for your dress with people who want to be there with you and who are excited for you. Eventually, parents have a way of realizing that grandchildren may not be far down the road and they want a good relationship with their own kids so a happy grandparent/grandchild relationship can be fostered. This is genuine (usually not manipulative) , and it is a beautiful thing when that happens. Your parents may never tell you they are sorry they missed your wedding, so I would suggest not having that expectation.