If you do a H1B transfer within the 180 days, you are taking a risk. The following does not say that you need to stay with the employer directly, but it is implied. Now if you transfer after 120 days and say you get a RFE on the 150th day, then you will have 2 to 3 months for the RFE deadline. By the time you reply the RFE your 180 day clock might have crossed. But you are starting to increase the risks of something going wrong.
Having waited for so many years to get a chance to file I485, why would you unnecessarily take a risk and go into a grey area? The whole point of filing for I485 based on DF dates is to avail AC21 benefits after 180 days. Anyway it is your choice.
USCIS Policy Manual has a Chapter on AC21. It says