I don't know if it is called " labour theory of value". I don't follow Marxism or any type of ideology, and I don't want to. Quality is often described by many properties of the product. Not just the amount of work effort but also how the work is performed. You also add aesthetics like look and presentation. That seems pretty much similar in all cultures including tribal ones. I don't think that some Aztecs, for example, had any idea about Marxism when they valued their artwork or craft.

I gave one comparison example above. You have (not talking about you as here btw) very interesting information to share with other but when you convert it to a post you don't care about proper editing removing grammar mistakes, misspellings and punctuation. You make it slapdash not even bothering to use capital letters when where they should be used.
It is like cooking very nice meal and serving it on filthy plate and dirty, rusty cutlery.
People are unlikely to be attracted to something like this as they see you as someone who is lazy or doesn't care.

It is not just about amount of time and labour you spend, as some people may be more experienced hence needing less time to do the same. It is about the properties of final product. It is also about using your own creativity and adding something personal and individual to it.
There are some products that you can just recognise that the least amount of effort was put into it and that such product could be easily made with minimum experience. An example is copypasta post of YT video. Anyone can do it. It requires almost zero effort, zero experience, zero personal input, zero creativity.

"that received a non-zero payout were exactly the ones I expected: one with practical advice, and one with Steem-related news"

I would advise not to look for such patterns in your posts trying to find out what information Steemit favours. I can assure you that there isn't any. If such content got rewarded, it was only because you were lucky. It is not because this platform favours such posts but quite likely because some more powerful user bumped into your posts aand liked it. Then he/she followed your similar posts bringing a squad of other whales who automatically follow his/her trail. There are many users posting great "practical advice" but get nothing. You are lucky with these posts that's it. Be aware that any time you may lose this trail of votes on that content.

EDIT:
You have had only 2 of your posts getting larger reward. You shouldn't try to look for patterns or make assumptions. This is not enough evidence and you have not been here for long.
On one them you got upvote from blocktrades. Blocktrades is a persistent curator who upvotes anything for his profit (his wife does most upvoting for him) regardless of content. They just auto-insta click-click-click on anything in feed which is in 15-30 minutes time mark for maximum profit. They never check the content. Steemcleaners has constant problems with them curating plagiarism and scams. There is a whole army of Eastern Europeans, mostly Russians who every day pop up trash content and plagiarism to fish for blocktrades votes. Steemcleaners cut one head off and another comes up. It will never end as long as blocktrades incentivizes it.
That was simply random.
It is low quality post: 3-sentence news and the picture. Typical type of post being created by new users.
https://steemit.com/steem/@edb/usd10-million-for-cryptocurrency-fund-interested-in-steem

The second post got upvote from val-a. There may have been something in the post (probably in the title and picture) that that user liked.
This one is a good quality post. Nice and personal.
https://steemit.com/advice/@edb/how-i-underestimated-the-importance-of-networking

Thanks for liking my posts, btw.