I have set a light to be on-off only using configuration.yaml but group light setting can override.

[feral saffron]

Using configuration.yaml, I have set a dimmable light switch to not be dimmable:
light.upstairs_bathroom_ceiling_light:
supported_color_modes:
- onoff

However, the group it is in can still set a dimming setting. This is bad for the light.
How can I get the group to respect the setting?
Alternatively, how can I trigger on changing the brightness of a light to change the brightness to 100% or 0% (high % or low %, like >25% keep it on, <=25% turn it off) ?

[feral saffron]

alias: Secure Upstairs Bathroom Ceiling On Light level
description: ""
triggers:

• trigger: state
  entity_id:
  • light.upstairs_bathroom_ceiling_light
    attribute: brightness
    conditions:
• condition: device
  type: is_on
  device_id: b55e06fbe699a0cb718d1b882b53fe7a
  entity_id: a90366e56432cb0579572685de65ee53
  domain: light
• condition: numeric_state
  entity_id: light.upstairs_bathroom_ceiling_light
  attribute: brightness
  above: 0
  below: 254
  actions:
• action: light.turn_on
  metadata: {}
  data:
  brightness_pct: 100
  target:
  entity_id: light.upstairs_bathroom_lights
  mode: single