Both gravel bikes and audax look more like road racers complete with bald tyres and curly handle bars - no thanks 
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From the description of big nobbly tyres it sounds like what we call 'fat bikes' which are basically weak looking frames attached to huge wheels? Probably a standard frame but compared to the wheels they just look weak, much like the 'hybrid' I tried. I've been reading that a big half flat tyre would probably be enough 'suspension' for the path but not sure it would like the pavement to get there?
Currently considering a very tidy looking 2008 Stumpjumper Elite but it's 26' wheels and only 2x9 gears. I wonder which ones they chopped off...
I've been seeing these fat bikes around lately and I don't get it. I'm not sure what they're for, but damn it's not commuting! Perhaps being towed up a mountain by a ski lift and then rolling down. I don't see what else they'd be good for. I have zero interest in this gimmicky crap.
As far as knobby tires go, a lot depends on the surface you'll be traversing. For the most part a good 32-42mm road type tire works on pavement, gravel, and hard packed dirt. Knobs only really come into play if you're facing mud. Otherwise they'll just slow you down. 30mi a day on knobby tires is a lot of extra work.
As far as the curly bars on gravel bikes go, I'd at least try one or two. While they may *look* like road bikes, it's all about the position the bike puts you in. There are plenty of good gravel bikes that put you in a comfortable upright position when you're on the tops of the bars. That's where you'll spend most of your time. In any sort of bike you intend to commute on, look for a bike where you can put the handlebars at seat height or higher. If the bars are a lot lower than the seat, you're going to be in an uncomfortable position where you're neck is craned, and there's a lot of weight on your hands and wrists. You should be able to support yourself in your riding position with no hands. If you can't do that try a different bike, or a bigger version of the one the salesman handed you. Here in the US anyway, most salesmen size you for a "racing" fit- basically the smallest frame you could possibly fit on. What I want in a commuter is more of a "french fit"- the largest frame I can standover without crushing the family jewels. I suppose if you're a lot younger than me this level of comfort matters less, but at my age I have no pretense of being the next Chris Froome. I just want to get there eventually and enjoy the ride.
Gravel and audax bikes also fit all the accessories you're going to want on a commuter that don't fit well on a racing bike- fenders, racks, lights, and bags. This stuff matters more than you might initially think. It's about fitness for purpose. If the bike doesn't meet all of your requirements, you'll have an excuse every day. No bag or rack, can't bring my laptop and a change of clothes! No fenders and it's raining- I'll get all muddy! No lights, it's too dark, unsafe in the winter! Knobby tires- too slow! Pretty soon that bike's been sitting in the corner for 6 months and you've wasted your investment.
As for bars, where the drops come in for me is in headwinds. The first time you get caught in a gusting, freezing, rainy headwind that stands you up and 1/2s your speed on your nice upright commuter, you'll be weeping bitter tears about your lack of drops which allow you to get low and tucked, just for the next couple of miles, just so you can get home out of the rain!
The other thing drop bars give you is several different hand positions. 15mi is a long way to hold the same hand position.My city bike has swept back bars (because it looks cool!) but after 5mi or so I start grumbling, wishing I had drop bars so I could switch positions because my left wrist is going numb.
