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.

I gave podman a try a few months ago and went with systemd unit files instead of compose files, since it felt like the most native approach.

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.