There are probably multiple ways to solve this…

Here’s an approach I’d choose (note that I do not use Alexa, so my knowledge about the integration’s capabilities is limited):

1. define an input datetime used as an alarm time.
2. allow Alex to manipulate this input datetime
3. use a template trigger to compare the current time with the defined alarm time - and yes you can do calculation; e.g., automations triggers when current time = (alarm time - 20 minutes)
4. trigger the alarm from within home assistant by either using a delay in the same automation or use a separate automation triggered at alarm time.

You mean that you just want to have one configuration entry? I doubt that it’s possible.

What you could do is creating multiple entries, but you need filter within a template.

{% if value_json['tags'].path == '/mnt/hdd' %}
    {{ (value_json['fields'].used_percent | float(default=0)) | round(2) }}
{% else %}
    {{ states('sensor.disk_use_percent_sdb3') }}
{% endif %}

Having had a similar use case, I ended up configuring device specific topics within telegraf.