If you are trading time delta, you would be giving up more edge to trade the less liquid options. However, you could be a master at legging in.....
As for trading FOP calendars to be short/long the front month vol, I would not do it on CL or ZS because time delta risk is too great and not clearly visible (if at all) in the greeks.
I would like to marry the vol trading and time spreading, but I get discombobulated over my exposure to volatility, correlations and expected return distributions of the paired strategies. Convexity in futures time spreads change sides. It is freaking confusing. VAR is nice. But, I don't want to be a deer in headlights when stuff hits the fan.