If you start the NAS without drives and insert the disks during setup, a full factory reset should be available as well.
I really couldnāt tell anyone what I did differently THIS time but perhaps I took the process a little slower & more carefully step by step as a THIRD factory reset + completely wiped HDDs (verified by Windows) seemed to proceed as I would have expected through a guided setup. The TS-216G initializing interface and sequence of operations seemed different than the previous attempts so thatās definitely positive! I certainly appreciate all the helpful feedback & suggestions throughout the process, thank you all!
*Note: Consider this topic/post resolved! 
Hi, glad to hear that the issue has been resolved!
Just a quick reminder: since these are used drives, we recommend checking their S.M.A.R.T. health status to ensure your data remains safe and secure.
Thanks for the update!
Thanks for response. Indeed as I noted above the HDDs in question are not too old & all S.M.A.R.T. status information is reporting as good/healthy. 
No need to verify your drives in Windows.
You likely missed the step of wiping the drives clean in the guided setup as you had (I am guessing) likely assumed they were fine and didnāt need to be initialized.
Definitely not a requirement, but was done as an additional step while troubleshooting where things went off the rails. I suspect I had mistakenly assumed the repurposed HDDs had been initially wiped & apparently that was not the case. Then I either missed a step during the QNAP initialization or there was an error when QNAP attempted it because the step-by-step initialization was much different than what I experienced the first two times.
Either way, Iām glad to have it resolved & also learned a few things along the way. Appreciate the responses/suggestions, thanks