Vegetarian sandwiches can be brilliant, but they need contrast. Cheese loves pickle. Halloumi loves sweet and sour. Falafel loves briny crunch. Egg mayo loves mustardy relish.
Try halloumi with achar. Try egg mayo with golden piccalilli. Try hummus and pickled cauliflower in a pitta. These fillings are simple, but the pickle brings the lift that makes lunch feel finished.
If you want to eat less meat without losing satisfaction, start with better condiments.