This is properly researched, especially the cholera run. Snow in 1854, the Great Stink and Bazalgette in 1858, then "where the sewers are done there is no cholera" in 1866. That's the real order it happened in, and making the player sit through the decade of doctors laughing at Snow is a great touch.

Also worth saying up front: the whole thing looks great, and the nerd-level detail is the best part of it, not something to apologise for. The Liberated Pixel Cup Victorian art was a smart pick and you have placed it with real care. It is obviously a lot of hours, and it shows. Nothing below is a knock, it is just the stuff you asked for.

Three things I caught. Two are date slips, the third I would argue is a mechanics fix.

Bandstand in 1837. The cast-iron park bandstand is an 1860s thing. The usual "first" is 1861, in the Royal Horticultural Society gardens at South Kensington, and once the Glasgow foundries got into it they spread fast. Before that a park band played off a raised wooden platform, often just called the orchestra. Your 1846 mission ("a Bandstand or Hedge Maze by 1849") asks for one more than a decade early. Gating it to about 1862 fixes it.

Fire-Escape Tenement in 1837. The building itself is fine, but the external zig-zag iron fire escape on the front is the anachronism. Those are a late-century feature and mostly an American one, coming in with New York's tenement laws after 1867. A tall tenement in 1830s Britain didn't have a bolt-on iron staircase down the outside.

The Water Pump gives +health from turn one, and that quietly spoils your best scene. Your 1854 entry has Snow taking the handle off the Broad Street pump because it was killing people. It sat next to a leaking cesspit. But in game a street pump is a flat health bonus from 1837 on. A shared shallow-well pump before piped water and sewers was a gamble: sometimes clean, sometimes draining the churchyard into the cup. If pre-1854 pumps gave happiness or convenience but not health, or carried a small risk near cesspits and tenements, then "The Mystery of the Pump" in 1854 would actually change how you play. Right now the player already had the health, so the reveal is just a caption on a bonus.

One maybe: 1839 as the first Grand National is the standard line, but Aintree ran the race in 1836 to 1838 too, and some historians count those now.