What i did more than once was directly get in contact with a mantainer or the lead developer(more often than not i befriended them too) and suggest some GUI improvements directly, it's ofc not doable everytime because otherwise the devs would just have too many people to manage.
Instead, ask if you can join the development team as the "UX/UI person" and show what works you already did, there is a good chance that you can either join or they will ear you out more often when you make an Issue/PR