Doesn’t collagen have more to do with skin? Mainly what i have heard from for example body builders that are cutting, if you workout your muscles while fasting you will maintain muscle and only lose fat. I’m certainly no expert though so you may need to do some more research.
Walking and doing small exercises to prevent muscle loss, however taking in anything “breaks” a fast. But if you’re doing it for weight loss just drink protein powder and fast everything else if you wanna go that route