maybe watch some videos on Floating thumb, and maybe moveable anchor.
The two extra strings wanna sing out too. You may also need to play with the bass much higher on your body to compensate for the extra girth on the neck and your wrist angle(s).
but I'd say "Happy Birthday" , "Twinkle Twinkle" etc..or play Cir of 5ths using octaves (all over the neck) to the beat of a slow metronome (drum track) click. till things sound clean.
Have fun bro (or ma'am)