Dunno if its any better than what you are already using, but I just use TV-Out + Interlacing preset. I actually made a thread about this on libretro, here are the answers I got.
That answer suggesting crt1-tap is new since last time I checked it.. might have to give that a try when I get home.
Problem is 480p is simply not enough pixels for most shaders to do their thing. If I was to upgrade to the RP3 this would be my main reason.