+1!
If you anchor your thumb on the body/pickup and not floating thumb or shifting to anchoring on a string, try:
- playing the same piece standing Vs sitting, noticing the different angles of your hand even though it's anchored to the same spot
- experiment with DIY anchors that aren't your pickups. My favourite are the little rubber chair or furniture leg stoppers you get from the hardware store. A small piece of chopstick stuck on with electric tape (that won't leave residue) works too. Trial an anchor position closer to the low E, and anywhere else
- consider the tactile feeling of your thumb anchor. If it's not comfortable it can trigger you to hesitate or be a little distracted. I've sanded the edges of pickups to address this.
Technique: to get your 3rd finger (ring finger) into play, just go back to basics. Play single note 16ths slowly, incorporating that 3rd finger. It's gruelling but works. Slow graduate to slow acdc songs. It doesn't matter what order or cycle you use your 3 fingers in, just let your hand and brain work it out without much thought.
I started out 1-2-3-1-2-3-1-2-3, then did 3-2-1-3-2-1-3-2-1. Now I can use my ring finger for low E and A string while my other 2 reach further for the high D and G.
Once you've worked on these, you'll be much more informed on a future bass purchase with ergonomics that you prefer. Or you'll be happier with your current bass