Okay, let's do that then:
Shield Master + Dueling: using the shield bash to shove your target prone: Adult Red Dragons have a strength bonus of +8. With Expertise in Athletics (which you can get from 1 level in Rogue) you can get your check up to +18, which means the dragon would have to roll 10 points higher than you to resist being knocked prone. The odds of that happening are 55/400, or 13%. So 87% of the time, we'll have advantage on our 4 attacks. Note that this is against a creature who's best ability score is Strength, against a creature with +0 strength, you'd have to roll exactly 1 while they roll exactly 20 in order for them to resist your shove.
Using a Longsword with Dueling, our damage output is 1d8+7, or 11.5. Using a longsword without Dueling, our damage output it 1d8+5, or 9.5.
With +11 to hit, we need to roll a 7 or higher, and since we have advantage, our odds of rolling at least one 7+ in 2 d20s is 92% as per the hypergeometric distribution calculator
View attachment 95115. So our average damage per attack is 10.58 with dueling and 8.74 without. With 4 attacks per round again, we'd need just over 6 rounds to kill the dragon with dueling and just over 7 rounds to kill it with defense. We've still saved over a round worth of attacks.
in the 13% chance that the dragon makes his check vs our shove, our odds of hitting fall to 14/20, or 70%. With Dueling, our damage per attack would drop to 8.05, and with Defense, it would drop to 6.65. That means we'd need 8 rounds of attacks with dueling and 10 rounds of attacks without.
going sword and board with Shield Master, dueling style saves you at least one round.
How about with Great Weapon?
Your chance to hit is now 40% (same math as for sharpshooter) in both cases.
A maul with Great Weapon Style deals on average 8.33+15 = 23.33 damage (the 3 repeats forever), where without great weapon, you're dealing 7+15 = 22 damage. Your damage per attack is 9.332 in the first case, and 8.8 in the second. Still using 4 attacks per round, it would take us 6.87 rounds to kill the dragon with Great Weapon Fighting, and 7.27 without. That's not as much as the other cases, but technically, we have saved one round of attacking.