Splitting a dashboard file into several using include statement or similar

Configuration Frontend
11

Best you can do is work in yaml-mode.
configuration.yaml → 

  dashboards:
    cl-home-km:
      mode: yaml
      title: churchlane KM
      icon: mdi:view-grid
      show_in_sidebar: true
      filename: churchlane/dashboards/ui-cl-km.yaml

(In my config km stands for kioskmode)
The ui-cl-km-yaml → 

base_theme: default
icon: mdi:desktop-classic
mode: dark

kiosk_mode:
  kiosk: true
  hide_header: false
  hide_sidebar: true
  
views:
  - !include /config/churchlane/navigate/woonkamer.yaml
  - !include /config/churchlane/navigate/keuken.yaml
  - !include /config/churchlane/navigate/beneden.yaml
  - !include /config/churchlane/navigate/boven.yaml
  - !include /config/churchlane/navigate/zonnepanelen.yaml
  - !include /config/churchlane/navigate/energy.yaml
  - !include /config/churchlane/navigate/verwarming.yaml
  - !include /config/churchlane/navigate/remotecontrol.yaml

Example remote control.yaml → 

type: custom:layout-card
title: RemoteControl
layout_type: grid
layout:
  mediaquery:
    "(max-width: 851px)":
      grid-template-columns: 100%
      grid-template-areas: |
        "menu"
        "main"
    "(min-width: 851px)":
      grid-template-columns: 50% 50%
      grid-template-areas: |
        "menu main"

cards: 
  - !include /config/churchlane/cards/card/stack-in/menu/menu.yaml
  - !include /config/churchlane/cards/card/stack-in/remotecontrol/menu.yaml

The menu/menu.yaml is a static menu that appears on every page I created → 

type: custom:stack-in-card
mode: vertical
card_mod:
  style: !include /config/churchlane/css/v4.0/vertical-stack-in-card-menu.css

cards:  
  - type: custom:layout-card
    layout_type: grid
    layout:
      grid-template-columns: calc(100% /6) calc(100% /6) calc(100% /6) calc(100% /6) calc(100% /6) calc((100% - 5px) /6) 
      grid-template-rows:    calc((100vh - 10px) / 8) calc((100vh - 10px) / 8) calc((100vh - 10px) / 8) calc((100vh - 10px) / 8) calc((100vh - 10px) / 8) calc((100vh - 10px) / 8) calc((100vh - 10px) / 8) calc((100vh - 10px) / 8)
    cards:

      - !include /config/churchlane/cards/card/weather/weather_1.yaml
      - !include /config/churchlane/cards/card/graph/energyprice.yaml      

      - !include /config/churchlane/cards/card/analog-clock/analog-clock.yaml
      - !include /config/churchlane/cards/card/button/menu/toggle_zonnepanelen.yaml

      - !include /config/churchlane/cards/card/button/menu/toggle_woonkamer.yaml
      - !include /config/churchlane/cards/card/button/automatisering/licht_aan_beneden.yaml
      
      - !include /config/churchlane/cards/card/button/menu/toggle_keuken.yaml
      - !include /config/churchlane/cards/card/button/automatisering/licht_uit_beneden.yaml
      
      - !include /config/churchlane/cards/card/button/afdekkingen/woonkamer/open.yaml
      - !include /config/churchlane/cards/card/button/afdekkingen/woonkamer/stop.yaml
      - !include /config/churchlane/cards/card/button/afdekkingen/woonkamer/sluiten.yaml 
            
      - !include /config/churchlane/cards/card/button/menu/geen.yaml
      - !include /config/churchlane/cards/card/button/menu/toggle_beneden.yaml
      - !include /config/churchlane/cards/card/button/automatisering/welterusten.yaml
            
            
      - !include /config/churchlane/cards/card/button/afdekkingen/keuken/open.yaml
      - !include /config/churchlane/cards/card/button/afdekkingen/keuken/stop.yaml
      - !include /config/churchlane/cards/card/button/afdekkingen/keuken/sluiten.yaml  
      
      - !include /config/churchlane/cards/card/button/menu/geen.yaml
      - !include /config/churchlane/cards/card/button/menu/toggle_boven.yaml
      - !include /config/churchlane/cards/card/button/menu/geen.yaml      
 
      - !include /config/churchlane/cards/card/button/afdekkingen/tuin/open.yaml
      - !include /config/churchlane/cards/card/button/afdekkingen/tuin/stop.yaml
      - !include /config/churchlane/cards/card/button/afdekkingen/tuin/sluiten.yaml  

      - !include /config/churchlane/cards/card/button/menu/geen.yaml
      - !include /config/churchlane/cards/card/button/menu/toggle_energy.yaml
      - !include /config/churchlane/cards/card/button/menu/refresh.yaml
      
      - !include /config/churchlane/cards/card/button/afdekkingen/slaapkamer/open.yaml
      - !include /config/churchlane/cards/card/button/afdekkingen/slaapkamer/stop.yaml
      - !include /config/churchlane/cards/card/button/afdekkingen/slaapkamer/dicht.yaml
    
      - !include /config/churchlane/cards/card/button/menu/geen.yaml
      - !include /config/churchlane/cards/card/button/menu/toggle_remotecontrol.yaml
      - !include /config/churchlane/cards/card/button/menu/beheer.yaml

Styling that repeated many times, I always include just one for one purpose. So you have to maintain only 1 file. And not every card on every dashboard.