Again, all you're doing is just making assertions without backing them up. "This is feasible. This is not." Why? You aren't defending your claims.
Conquering and ruling over literally the entirety of Planet Earth with the technology (including communications and logistics) of the 1820s is absolutely impossible.
What level of development should be considered "impossible?" That's tough to answer. What is a production unit? What does a ducat represent? These are extremely abstract game mechanics.
Manpower is less so, so let's focus on that. At 100 military development (and no buildings) you have a base of 25,000 manpower from a province. Assuming you're able to make 10% of the population into soldiers, that would be a minimum of 250,000 people. Quadruple that with tech and buildings and whatnot, and say you can't extract more than 5% of a province's population, and you're still talking about a population of around 2 million across all the cities of a province. That's pretty nice, but London was close to 750,000 in 1760 while Beijing had hit 1.3 million by 1821. We're not that far out of range.
Maybe the particular province you choose to develop that high is a silly choice, but player conquests tend to be equally as silly (whether it's a resurgent Byzantium, a Ragusa that conquers the Balkans and Italy, or Ryukyu conquering anything).
The most important thing is what the AI does with this. If the AI builds 300-dev provinces all over the world then yeah that's not going to work. If the player does it... who cares? The player could have spent the same monarch points on an equally silly conquest-fest.