Non resident Alien ?? about tax on 401k

I've lived/worked in USA for 20+ years, but never became a citizen (green card).  My Thai wife (of 20 years) and I will be moving to Chiang Mai in the early 2019.  We are both 65+.  We're starting our tax planning before leaving US.

I have a 401k and I am not sure when I will start withdrawing from it.  I also will receive an additional small pension from Canada.

I have basically two options regarding USA taxation.  I can arrange to be taxed as A) resident alien or B) non-resident alien.   The Foreign Earned Income exemption won't apply as non of our income (pensions and investments) will qualify.

Option A will result in my being taxed as if I still reside in US with a progressive tax rate schedule and I will certainly have to pay tax on 401k disbursements.

Option B requires that I surrender my green card and write a letter telling US gov't that I'm abandoning my Permanent Resident status.  I will then pay a fixed tax rate of 30%.  Article 20, paragraph 1 of the Thai-US tax treaty implies that individual retirement account disbursements will be taxed in Thailand at 0% (according to USNews).  I think that this is probably not true since my 401k contributions were pre-tax dollars.  I'm not 100% clear about exemptions and deductions for this option. 

Here are my sources of expat tax info:

https://money.usnews.com/money/blogs/on-retirement/2011/12/05/the-most-tax-friendly-places-to-retire-abroad

https://www.irs.gov/publications/p519 (US tax guide for Aliens)

https://www.irs.gov/pub/irs-trty/thaitech.pdf  (Thailand-US tax treaty)

Does anyone have knowledge or links to more accurate information?

Thanks.

Gamb00ler