That's not how portmanteaus work.
The reason I find this so interesting is that Mandarin Chinese portmanteaus take a different standard form: instead of combining all of one word with half of the other word, they combine half of one word with half of the other word.
Think about how much you'd need to know about the structure of an arbitrary language before you'd feel confident predicting how it creates portmanteaus.
It doesn't. You might note that e.g. wiktionary categorizes motel as a "blend", but sitcom as a "shortening".
"Blend" and "shortening" are necessary-but-not-sufficient synonyms for portmanteau. Your error here is that you imply "because it's a blend, it's not a portmanteau." All portmanteaus are blends. All portmanteaus are shortenings. Some blends are portmanteaus. Some shortenings are portmanteaus.
Looking at https://byjus.com/english/portmanteau/, because it was easy to find:
guesstimate, mocktail, popsicle, breathalyser, athleisure, bromance, frenemy, tragicomic, docudrama, webinar, Medicare, listicle, fanzine, scromelet, chillax, frappuccino, permafrost, staycation, workaholic...
guesstimate, bromance, and tragicomic include all of each of their source words.
And just by looking at your list, you can see why the norm is to include all of one word - those are difficult to interpret for English speakers. Brunch, spork, and sitcom are well understood, though sitcom is more likely to be seen as an abbreviation than a combination. Probably cronut will be understood too, if someone's looking at one. Motel is obviously related to hotel, plus an inexplicable m. Smog is guessable. Cyborg and Velcro are completely opaque; those are just "words".
count percent
both 5 6.4
first word only 29 24.4 (you said "usually," I said "rarely")
neither 39 50.0
second word only 25 19.2
By the way, I called the code "portmantotal.py" :-)
Cyborg was not opaque to me, because I grew up reading science fiction so was very familiar with the concept of the cybernetic organism. My parents explained the origin of "velcro" to me when I was a kid and thought it was cool that it had two sides.
EDIT: While playing with the data in python, I realized that chatgpt included 8 duplicates in the above list originally. I will fix that and recalculate the numbers.
Also, I was thinking about compound words, like seatbelt, schoolbus, butterfly and marshmallow, where all of both words is included -- I don't consider those portmanteaus. And yet, the "both" category in my list is interesting: "covidiot" works because the words overlap, and "blogosphere" adds an extra letter. "Manspread," in my opinion, is also a portmanteau but it's hard to explain why I wouldn't just call it a "compound word" . Probably either because it's derived from "mansplain" or because it adds a noun to a verb whereas compound words are usually noun + noun.