For derivative Adjustment of status (EB2)......I-134 or i-864?

I-134 with employment verification letter. You should retain an experienced immigration lawyer to review all the facts and advise you accordingly.

I would also include a copy of your most recent tax return as well as the employment verification letter and copy of approved I-140 as well as the other required forms.

First, it sounds to me like you may need to retain experienced immigration counsel at this point. You would need to file an I-134 in this context with all supporting financial documentation. Your employment verification letter is only one piece of the documentation which should be filed to support your I-134.