The toilet was originally sealed with a good water seal and had no smell, but now it is very smelly

Not necessarily, it could be that the toilet was designed to be faulty when it left the factory.

OK. As long as you don't change the whole house or the main structure, it's allowed! However, remodeling the restroom requires water and sewerage. >>>More

There will be various pollutants on the four walls that have not been washed, and over time there will be more and more pollutants, emitting a foul smell.

If this happens, I think you still have to reinstall the toilet, and if it affects the downstairs of the house, then you still have to deal with it.

Now there is no need to bend the pipe, there is a bent pipe in the toilet, after flushing, part of the water will be stored in this bend, so that the sewer pipe and the sanitary partition are separated, and there will be no smell, you haven't installed the toilet yet, just install it.