Visual Studio Code, an open source IDE with extensions from Microsoft.
Your Cabal project lives at ./xmonad-config … and, if you followed the instructions in Creating a new Haskell project with IDE support using Nix to setup IDE configuration, you can simply launch VSCode using code ./xmonad-config to start editing your configuration.
My next computer, if I choose to buy one in ~3 years, would likely be similar to the X1C7 but with a bit more performance (assuming battery life does not suffer); i.e., if I were to make this decision again, I’d consider Thinkpad X1E or P11 - but with integrated graphics (nvidia has poor support on Linux). That said, I still use the Carbon as my primary computer, and use VSCode remote to shift much of the develoment heavylifting to the P71 workstation at home.
And if Thunderbolt wasn’t a requirement, I’d easily go for a Thinkpad with AMD processor, such as the T14.
The carbon does suffer a bit with heavy workloads, such as some long compilation (eg: GHCJS) tasks or IDE heavylifting (haskell-language-server). However, I can offload them to my P71 workstation at home (via manual ssh, VSCode remote ssh or distributed build), and then use its binary cache.