What can be used in place of Tofu?

If you can't tolerate a lot of tofu, leave it out and increase the mushrooms and/or noodles. Maybe even add some noodles for more veggies.

I agree, that's a ton of tofu and I think you could just cut it down, adding a few more noodles if you're worried about it being filling enough. I like tofu quite well but I'd probably put only 1 14-oz block of tofu in a similar batch of soup at the most. I also generally prefer a softer tofu than extra-firm in this kind of soup.
You could add beaten egg to the soup at the end, as is common in this type of soup. I have to disagree with Brinda that tempeh would be a good substitute, however. I don't think tempeh's texture does well in brothy soups at all.

Hi Peggy! Seitan, tempeh, chicken, or even eggplant could work here, I think. Just want something with a firm texture that won't totally fall apart after being slow cooked for that long. Let me know if you try it!