Your strategy seems to somewhat work if you don't get around to building the palace until later in the game. This approach works well on smaller maps, and also if you plan on a lot of conquest. On a tiny map, I used the Germans, on the right side, to take over the French over time, and built the Forbidden Palace in Paris around the same period as you. The French had already done the hard work - building roads/mines/irrigation - so I just needed to rush some crucial improvements.
However, in the larger games, I think another strategy bears mention. I know I've seen it mentioned early and often, but I think it bears repetition. Let's say you have an average sized Civ, with about six or seven core production cities. Your capital, then two or three around it, then a couple more a little further out that have done pretty well. You've just switched to Republic, but there's still some corruption on the fringe. In addition, those pesky Zulus to the north are itching for a fight, and you want to lay the smackdown on them.
Rush build any remaining cultural improvements - you're in better shape if they've already been built - you can just start on a palace whenever you're done with those and keep going until you switch to Republic for some extra shields. Be sure you're connected to the capital and luxuries and are either on a river or have the Aqueduct completed. And get some mines in place.
Once you're in republic, you should see a corruption drop-off. The extra productivity, combined with saved shields from the palace, should get the puppy built in 20-35 rounds. Best part is that when you are done, you'll boost up the bulk of your "early empire." And have a cultural boost to help take some of those pesky neighbor's cities for free, while strengthening your own culture to prevent a flip when you launch your actual invasion.
Now here's the trickier part. After you switch to Democracy, you can use Zachriel's Forbidden Palace trick to build your real palace in an opponent's capital city. If you're on a huge continent, you can build it on the other end; if you like the way your own continent is, you can build it on the other continent. I know of no other way to effectively maintain an empire on another continent, short of this strategy. The corruption is too great otherwise, and it's very expensive to rush build everything in every city. But a well placed palace after you rush build in just one city works well. And you don't have to save your forbidden palace for the late game.
