It could be a number of things.

There may be insufficient down thrust -- you can test that by shimming the engine mount.

It could be an incidence problem. You can test that by shimming the trailing edge of the wing upward with a tongue depressor under the the wing where it fits on the wing saddle.

It may be tail heavy -- you are addressing that already

The ailerons may be incorrectly configured. The aileron linkage should be adjusted so that the trailing edge of the ailerons are NOT depressed below the trailing edge of the wing. Try trimming them so that they are reflexed slightly above the trailing edge of the wing.

Only do one thing at a time.

Good luck & get back to us.