CDM - I'm not sure that the OP is here seeking those types of relationships for themselves, but curious how anyone who has FWB or FB maintains those relationships.
I know some poly people who have relationships as well as FWB. Personally, I have let my initial FWB revert to a friendship and grew my second one into a full blown relationship. Initially, it seemed that Wendigo's wife was okay with us being physical as long as we stuck to foreplay and no PIV sex. Then when we realized there were emotions involved and did have PIV sex, she asked that we suppress the emotions but was okay with the sex. That went on for a 6 months before she and I sat down and came to an understanding and our relationship started progressing.
With my initial FWB we only saw each other a handful of times a year and while we remained good friends in the interim though chat, text, and phone calls. I was never interested in an LDR. The sex was fun when it happened, but I could take it or leave it and didn't miss it when he was away so I never felt the need for it to move to a different type of relationship with him.