Your description of needing full down elevator while inverted makes me think the model may be too nose heavy. If you have an instructor or other skilled pilot available, ask if they will fly it and see if the battery needs to be moved back to adjust the CG.

A note of caution. If you do adjust the CG rearward, the elevator will become much more sensitive. It will almost seem like you've gotten a new plane. You might even consider asking for a buddy box flight, or two, just so there is a safety net in case it seems too sensitive at first.