Thank you so much for your answer. 

Based on your advice, I did some more research and now I am almost certain FIFO and LIFO (even HIFO) are acceptable to IRS.
 
https://gritdaily.com/irs-releases-new-rulings-cryptocurrency-taxes-2019/
https://donnellytaxlaw.com/tax-expert-picking-the-best-method-for-reporting-your-cryptocurrency-gains/

Koinly allows me to compare FIFO, LIFO, HIFO for my capital gain.  For me, LIFO was about 1/2 of FIFO. 

Thank you.