You can also try doing one restaurant meal a day which would be very Italian of you. So have no breakfast or a small breakfast, good lunch, and go for a before dinner aperitivo with couple of plates to share. Then stop for a gelato or crepe later in the evening or maybe a focaccia or pizza to share if you are all still hungry. Maybe your hotels will have a balcony or somewhere that can serve as a picnic area for you.
You may also find everyone is hungrier than normal if you consider you will all be on your feet a lot touring during the day, the fresh air, and the amazing food.
If you don't think this would work for you, then my advice is to eat a little earlier and/or not at a crowded restaurant. Be honest with the staff before you sit down "we would like to eat but aren't very hungry and will just be sharing a couple of things, do you have a table for us?". If they are expecting a crowd, they will politely tell you they are full for the night. If they aren't, they will probably be glad to have you rather than an empty table. And if you are turned away but you really want to try that restaurant for some reason, ask if there would be a better day to return.
I have never eaten like a bird so can't relate to the portion size issue but I do travel solo a lot. And I am mindful that busy
restaurants will be hesitant to give a table to one that could be a table for 2-4. So I stick with the above plan and I don't dawdle. I eat at a comfortable pace but I don't linger too long at the table so they have a chance to turn it over.
I have faith you will be able to enjoy your trip. I am also crossing my fingers for myself for a late September departure.