But building a new wallet even with an advanced GUI is usable?
Can you link me a guide for it please?
you're welcome.
i think it is useful because although we have very good wallets but many of them suffer from a lack of user friendly interface. take bitcoin core for example. it is capable of doing a lot of things but 90% of them happen through using the command line and that is not user friendly.
you may not want to start from scratch and it may be better that you contribute to an existing project and help improve that. if you look at the code of these big projects you can gain a lot of experience that guides won't give you in my opinion.