IMO, although I understand your point about using your miles before a potential devaluation, I would still considere buying the R/T ticket in cash. I do not know if it works for your fixed dates, but I see R/T trips in Y for about 450€/500$ in September, including on AF which would prevent your FB miles from expiring.
Keep your miles if you need to buy a one-way ticket or need flexibility for a future trip, which does not seems to be the case for your planned journey in September.