Since, you have an 80% win rate and only a 5% return on average, what happens when you have a couple of large losses because a straddle takes two positions a once? So, it costs you more for a single trade compared to a directional options trade? Wouldn't a couple of large losses wipe out any small gains you made? It is not the winning percentage that matters but, your edge. What is your win/loss ratio? How large are your wins vs how large are your losses? If you have a 2/1 win loss ratio and say 40-60% winning percentage on directional option trades, you should be fine.
Our largest loss in 2018 was 11%. In 2017 we had two losses in 20-25% range, the rest were single digit losers. This strategy has very low risk. Average winner is around 8-10% and average loser around the same. Since winning ratio is around 80%, the strategy performs very well. We don't do any directional trading. Just don't need to take all the added risk since our non directional strategies perform so well.
The trick with long gamma is to kill the trade/hedge at extreme moments (or inflection points) on the vol curve or vol environment. It’s not easy, but very possible to make money always being long vol, even in a low vol environment.