When you have an adult, the correct weight can vary because dachshunds can be 8 lbs or go up to 35 lbs. The best weight for your dog is when there is a waistline. The chest will come down, then the waist should rise and tuck up.
If your adult dog looks like a tube, they are too chubby.
The chest takes some time to drop so in the first year they will look like a tube, and that's ok.
It's better for adult dogs not to be free fed. Dachshunds have a tendency to eat as much as you give them, so measuring kibble is very important. Treats don't have to be store bought, they will enjoy things like apple, strawberries, carrots etc, and those things are good for them. Store bought treats can have a lot of fat and calories so better not to get into that habit. There has been a lot of recalls on those treats too, just avoid them from the start.