The idea that warfare used to be romantic and gentlemanly is one of the largest wives tails in history.

During this time period, a lot of the tactics used on the battlefield had to do with the weaponry of the time, as well as the geography of Europe. Many have always talked about how soldiers lined up in formations and fired at each other point blank to show how it was "romantic and gentlemanly." That had nothing to do with being gentleman. It had to do with the muskets at the time were not rifled, so therefore they did not have great range. They were also notoriously inaccurate. The gentleman were in formations firing at each other point blank because that had the most firepower as a unit giving them the potential to kill more people. Also, at this time, Europe was very much agricultural based. So, a lot of the forests had been chopped down. So, you had great expanses of open fields to maneuver in.

You mention no ambushes. That is incorrect. The tactics of the time changed here in North America. Because this area was so tremendously forested, the British had plenty of hard times adapting to new warfare that came along. During the French and Indian War ambushes were prevalent and also during the Revolutionary War.

You mentioned that troops faced no bombardment. Once again that has to do with weaponry. The cannons during this time period were once again not rifled. So, they could not bombard in the tradition sense. Most of the cannons of the time delivered flat trajectories. But this also throws the romantic and gentlemanly concept out the window. One common tool when enemy infantry came into close quarters was using grapeshot. A combination of pellets, or just any kind of junk found to use as shrapnel. I've heard accounts of cannoneers loading their guns with forks and knives. They'd fire these guns point blank at enemy troops blasting holes through the lines.

There has never been a point in human history where warfare has been gentlemanly and where someone is not trying to figure out some way to outkill the other guy.

Ok OtherCountry, meet at that big field over there at noon

You'll excuse me if I'm unaware that this was ever the state of affairs in 19th century (or any century's) warfare. Declarations of war were followed by mobilization, and the marching of armies; battles were often encounters between one force and another; "Winner takes Prize" was rarely the result of Battles, save for perhaps in exceptional cases like Ulm, Austerlitz, Sadowa and Sedan (and the French fought on for a year after Sedan!)

No tech made after YearXYZ

The only technological restrictions I am aware of were the Hague Conventions, which took place at the end of the 19th century, and mainly focused on things like poison gases and submarines. Beyond that, European Armies embraced rifles, breech-loading artillery, even rockets, and used them to deadly effect.

2500 men each

Suffice to say that even before the Napoleonic Wars, major battles between European powers were rarely that small. By the end of the Napoleonic Wars, Europe had seen battles involving armies in excess of 100 000 men, which lasted for days; see Wagram, Borodino, Leipzig, Waterloo, etc

seems like a lot of conflict could be resolved quickly and effectively if this were the case

The Wars of the Coalitions were extensive affairs, as were conflicts like the Seven Years War before them. Even after that, wars like the Crimean War, the Italian Wars of Unification and the Russo-Turkish war involved at least a year of fierce fighting. Even the six week Austro-Prussian War involve a fair amount of maneuver, and a good deal of carnage.

Most of the conflicts in 19th century, even if they were short, were bloody affairs. Conflicts such as the American Civil War, and the various conflicts that took place in South America, followed a similar trend.

Other users should be able to add to this, but I think you might be going off a number of misconceptions about the realities of warfare in the 19th Century.