Attributes can be longer than 255 characters; perhaps you could put the URL into an attribute instead, and some dummy value for the actual sensor? Or some other response (possibly even a truncated string) that you can ignore.
You can use the json_attributes thing to capture this other info.
Take a look at the file I attached to the first post in this thread
Monitor your Generac generator with Home Assistant for an example. Just an excerpt that might give you a sense of this:
- platform: rest
name: genmon_status_json
hidden: true
resource: http://generator.local:8000/cmd/status_json
value_template: '{{ value_json["Status"]["Engine"]["Engine State"] }}'
friendly_name: "Generator State"
json_attributes:
- "Status"
- "Line State"
- "Last Log Entries"
- "Time"
method: GET
scan_interval: 65
and the template sensors that look like this:
- platform: template
sensors:
# outage related sensors
genmon_outage_status:
entity_id: sensor.genmon_outage_json
friendly_name: 'Generator Status'
value_template: '{{ states.sensor.genmon_outage_json.attributes["Outage"]["Status"] }}'
which access the attributes that were previously squirreled away.
Perhaps something like this could be adapted to your problem?