No Right Answer.
As for being out of breath .... yes, in general, spinning faster puts more load on the cardio systems, and spinning slower puts more load on the muscles (because in part, the cardio system has to flush more frequents spurts of waste because you are pedaling faster) but no matter what, you are using your muscles continually for 45 minutes ..... are you thinking it would be easy, climbing that 20% hill.
Taken to the extreme (reductio ad absurdum) if you only pedaled one stroke per minute you wouldn't get tired and would just float up the hill, right? Sorry, but climbing a 20% grade Walking is tiring. Cycling it is tiring in a different way.
Personally, I would think 40 rpm would be too low to be efficient, because you would lose too much momentum between pedal strokes---but you made it. 60 is definitely grinding, but some riders prefer that. You have to listen to your own physiology (and remember, you are taking hill-climbing advice for m a rider who is challenged by three percent for 700 meters, forget 30 percent for seven km.)
Personally, if that climb were part of my regular route, I would replace the 11-34 ... but I couldn't make that climb with an 11-46 or 10-51 or any gear, so ..... I do like the 11-28 for most terrain because it goes low enough (for me) to make the hills and is nice and tight in the small-mid cogs .... but seven km at 20-30 % is a well, 45 minutes and that would take a lot out of me.
Generally speaking, based on your one short post (what doctor wouldn't prescribe life-changing medical regimen to a guy he met online after two paragraphs of description?) I'd say toughen up until you can keep the revs above 60 .... using whatever gearing you needed. But if you are strong enough (joint-wise, not in terms of muscle) to ride it with 34x28 .... good for you. I would worry about knee strain, but if you are paying attention you might be fine.
Or you might post a week from now about how you need double knee replacement. I wash my hands of all legal liability.