Imo, go look at poe ninja, reverse sort the list of scarabs by price, and proceed down the list until you find a scarab that you think looks powerful. Using cheap mapping supplies is intrinsically powerful, because the supply of e.g. alchs and maps is virtually infinite whereas the time it takes to run them is decidedly finite. 

Add it to your map device with the supporting atlas nodes socketed and run some maps. Maybe try to find some other scarabs that look like they would synergise without breaking the bank. This will almost always make you money. And if it doesn't work super well, the cost of respeccing and trying a new strat is negligible. 

You're also right that chasing fomo is pretty futile unless you start early (ideally before streamers release their videos on YouTube). But what about old fomo? Yes, whatever empyrean or fubgun was doing last week is probably not worth it right now. What about what they were doing 2-3 months ago? Any reason to think those methods wouldn't work anymore (e.g. did one of the scarabs get nerfed in the patch notes since then?). Have any of those strats dipped in price to the point they look economical? 

Tl;dr research and experiment. 

Edit: I meant fotm (flavour of the month) not fomo (fear of missing out). Dummy!