E.T.
R. - what a neat concept for a series of seminars. I hope you have tons of people attend! I think you could offer sandwiches each week and leave it at that. Maybe a platter of various sandwiches, two different salads, and cookies. That's really enough for a day long meeting. I go to seminars all the time for work and the hot foods are nice (like a chicken dish or beef tips) but people seem to like pasta salads, potato salads, green salads, and something simple like cookies for dessert.
Soup is REALLY hard to eat at a business type meeting. People end up leaning forward awkwardly, especially if you have just open chair seating and not tables. As for vegan, vegetarian, gluten free, dairy free, etc.... ask people when they register what options they need and then tailor based on that. If you're expecting 25ish people each day, it would be hard to have all of those varieties and still leave enough "regular" choices for people.
You didn't ask for advice on pricing... but I think your proposed costs might be a bit high. I've never attended anything like that... but if you can cut down your food costs a little you might be able to reduce the price slightly. Good luck!!