Wow this is fantastic! Glad that you managed to pick up the FL980 - I’m tempted to pick one up myself! I just got some “Unholy U4s” (U4 stems in Halo housings) and am also thinking of a board to put them in 
In terms of the keycaps, the stock ones are in Cherry profile while the orange set you got is in OEM profile, hence the differences in contours. One way to remedy this will be getting a single color shine-through keycap set with compatible modifiers, and use those for a two-tone colorway. Something like the Tai-Hao set I’ve linked below. If you’re not keen on the PBT Oliva Dark keycaps you got there, you might be able to find some luck selling them on the aftermarket (r/mm, ebay, your local discord etc).
It might be a little hard to find a full set of orange shine-through keycaps with 1800 compatibility. The important compatibility points are:
- 1.75u Right Shift key
- Three 1u bottom modifiers keys
- 1u numpad-0 key
But if you’re interested, I managed to find a couple of 1800-compatible orange-themed keycaps:
