Qt5 apps ignore default font unless plasma6 is enabled

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As far as I can tell, any qt5 app (ksnip and qsstv in particular are problems for me) will ignore any default font setting, set using qt5ct, qt6ct, or fonts.fontconfig.defaultFonts, and use the first installed font alphabetically, unless all of plasma6 is enabled. In this case that’s Air America which I installed by putting the otf in ~/.local/share/fonts and running fc-cache.

This only happens with qt5 apps.

I’m assuming it has something to do with plasma installing qt components I don’t otherwise have installed.

qt5 and qt6 apps also don’t properly fit gtk theming without plasma installed, so I imagine that’s a similar issue, but I can usually live with that

Relevant sections of config:
configuration.nix

{ config, pkgs, lib, inputs, ... }: {
  imports = [
    ./hardware-configuration.nix
    ../../features/nvidia.nix
    ../../features/sound.nix
    ../../features/gnome.nix
    #../../features/plasma6.nix
  ];

...

  fonts = {
    packages = with pkgs; [
      iosevka-comfy.comfy
      atkinson-hyperlegible
      departure-mono
      noto-fonts-cjk-sans
      noto-fonts-cjk-serif
      noto-fonts-color-emoji
    ];
    fontconfig = {
      enable = true;
      defaultFonts = {
        serif = [ "Liberation Serif" ];
        sansSerif = [ "Atkinson Hyperlegible" ];
        monospace = [ "Departure Mono" ];
      };
    };
  };
};

features/plasma6.nix

{ lib, pkgs, ... }: {
  #services.xserver.displayManager.sddm.enable = true;
  services.desktopManager.plasma6.enable = true;
  programs.ssh.askPassword = lib.mkForce "${pkgs.x11_ssh_askpass}/libexec/x11-ssh-askpass";
}

home.nix

{ inputs, config, pkgs, lib, ... }: {  
  qt = {
    enable = true;
    platformTheme.name = "gtk2";
    #style.name = "adwaita-dark";
    # this only half works, results in unreadable half dark half light qt6 apps
  };

  gtk = {
    enable = true;
    #theme.name = "adwaita-dark";

    #gtk2.extraConfig = ''
    #  gtk-color-scheme prefer-dark
    #'';
    #gtk3.extraConfig.gtk-color-scheme = "prefer-dark";
    #gtk4.extraConfig.gtk-color-scheme = "prefer-dark";
    # these do nothing afaict
    
    iconTheme.name = "Adwaita";
  };

  home.packages = with pkgs; [
    ...
    qsstv
    ksnip
  ];
};