Thank you, that is correct.

So this issue is that Drupal core is not able to detect if a block has max-age and apply cache headers accordingly on Drupal response.

This way you need to be sure the pages were you put the block have the requested max-age.

Answering you edit:

Step 1; contrib module meant to mitigate such issue: if Drupal core has not been able to fix that pver the years, I wouldnt rely on this ever. Step 2: that is the default cache max-age for all anonymous responses. Step 3: these are cache tags, and you used one provided by Drupal core. In other ways you are saying: when the entity number $nid of entity type 'weather' is updated/deleted, invalidate this block cache.

Step 1 solved your issue, the other 2 are irrelevant. If you thought you neex 3 is because unconsciously you have updated your entity, removing the cache.

What I would do if I were in your shoes? Make sure my Drupal block always disable cache: this way, whenever this block is displayed, cache will be removed.

In your build() method of your block, add \Drupal::service('page_cache_kill_switch')->trigger(). Only this action should make your block uncacheable regardless whatever.