What's the root concern? Cost, imbalance, or inconvenience?
- If it's cost, she can pay for herself.
- If it's imbalance, don't go 50/50. She can pay for whatever she wants.
- If it's inconvenience (takes too long to order and get food), you can order and ask for your food to be brought out when ready. Or just wait, chill out, and enjoy some time together.
You should never worry about inconvenience to the restaurant or staff. Substituting is normal. If they can't do it, they'll tell you. Otherwise they'll happily provide the food and take the money. That's the social contract.
From your description, it looks like the main issue is #1 and #2. If so, a frank conversation should fix it. It may solve the problem, or uncover larger issues and expectations you should probably handle before getting married.