On road, gears much less than 1:1 aren't really quicker than walking anyway. 4.8km/h is a realistic walking pace pushing a bike uphill. With 30:40 you'll ride this at a low - but not unreasonable grinding up a hill - cadence of about 50rpm. There even comes a point when balance is the limiting factor.
It may be that your comfortable cadence is rather high; then you would benefit from lower gearing than me (I'm happiest at around 80rpm, which is rather low). A higher cadence in the same gear means more power, so if you're at your upper power limit and your lower cadence limit, a lower gera would help . I think you've already got that with 30:40.
Some points of comparison
While I haven't done a 1500km ride, I've done 750-800km in 3 days a couple of times, with a light touring load. The bike I used for that has a lowest gear of 30:32, and the one I've used more recently for a hilly 600km in 2 days has 30:30. My friends who have just got back from Paris-Brest-Paris (1200km in 90 hours) have similar gearing, down to 30:34 in the lowest case I know of - and a few were on fixed gears.
I struggle with my back if I pull up on the bars too much, but find standing unreasonably tiring, so do sometimes walk the steepest bits (>15-20%), particularly if there's a lot of climbing to come the next day. You sound like standing to climb has even more of an effect on you than on me. But 10% should be sustainable seated, just standing for a few strokes if there's a sudden extra-steep bit.
Broader aspects of multiple long days
The difficulty with such long days back to back is that recovery time is very limited. So as the ride wears on, you're starting each day in a worse state than the one before. Even if you don't get much rest, plenty of food is a must, and food, a bit of sleep, then more food is a good way to do that.
The load and the headwind combine to really limit your on-bike rest opportunities (and off-bike, because you're in the saddle for longer). That led to me cutting short one trip. Aerobars help with the headwind flat sections, and a few people climb on them (not me).
Putting these together, if your reserves are depleted and you have to keep climbing, your power will be low, meaning a low climb rate, and a low speed
The most training most of us can do for something like this is to get a whole weekend at the same or slightly higher daily distance. The odd longer weekend helps a lot.
Fasted training may also be beneficial here, as it improves your power output when nutrition (in the form of carbs) is lacking. That could be a stretched commute of 30km before breakfast, once a week. If I was training for something like this, after a weekend of 400-600km, going out fasted (or with a protein shake) on the Monday morning would be brutal but realistic training, also for the psychological challenge of getting going again.