My thoughts as a Fertility Awareness Educator:
With typical use, Phexxi efficacy is similar to using just condoms. (~86% effective in preventing pregnancy.)
Doubling up on birth control is great. I personally like to use two of the following when I'm fertile: diaphragm, condom, withdrawal. Phexxi could definitely be another combo option.
I'm thrilled that there's another non-hormonal contraceptive on the market. It could be a great method for someone avoiding synthetic hormones, and could be used in conjunction with FAM.
With any type of birth control, read the instructions and follow them perfectly.