I also used to get very frustrated with WordPress. So eventually, I dropped it. But I do seem to recall some of my child theme issues were solved by being meticulous with version numbering. Because WordPress calls the child theme with the version number: https://laspina.org/wp/wp-content/themes/2019-child/style.css?ver=1.0.0

So that's caused me caching issues in the past. But I don't think that is the problem here. If I interpret your issues correctly, they are:

1. Remove Twenty Nineteen's mini rule above our tag subheading
2. kill extra gap under the subheading article

The 'mini rule' is actually a dashed border on h2. So you can override this in many ways. Personally, I would just delete or comment out the original definition, border-top: 1px dashed rgba(149, 114, 169, 0.4); But I kinda like your self-documenting way of keeping the originals in place. So maybe just add border: none; to your override?

The extra gap is mostly down to padding-top: 1em; in the same definition. So you could override it with zero.

In my screenshot, I just disabled the offending rules in my browser. Is this what you're after?

!BBH