There are two different approaches to this issue:
1) make your own butter and use the resulting buttermilk; or 2) if you're not satisfied with the cultured buttermilk on the market, modify it.
1. Making butter is pretty easy - to experiment, start with about a pint of heavy cream, with as low a level of pasteurization as you can get (raw if you know a friendly dairy farmer, but probably the best you can find easily will be HTST, which works pretty well. I'd avoid anything that's UHT or mentions carageenan or other stabilizers.) Then to be really authentic (as opposed to mass market butters these days), add lactic acid bacteria to the room-temp cream, and let it sit out overnight (covered) at room temperature. (Where do you get lactic acid bacteria, you might ask? Why from cultured buttermillk, of course! Why do you think those specific cultures are chosen to make cultured buttermilk?) Then whip it (I would use regular mixing/beating methods – e.g. the paddle attachment on a Kitchenaid stand mixer rather than a whisk, or maybe a food processor – unlike making whipped cream, incorporating air isn’t a priority here – it’s the effect on the milk protein structures). It’ll get frothy, then like whipped cream, then grainy, then finally you’ll have a mass of butter in real buttermilk. Neither the butter nor the buttermilk will last as long as commercial products, but both can be frozen for storage of more than a week or so. Expect the buttermilk’s consistency to be equal or slightly thinner than today’s commercial product. You may or may not be able to detect taste differences between commercial and your own, and if you do, it’s a toss-up re: which you’ll prefer if you do a truly blind test. And, your homemade product will probably be between 1.5% - 2% butterfat – if you wanted to sell it commercially, you’d have to label it “Lowfat buttermilk” – unless you went back and added cream to raise it to 3.5% butterfat. But then it wouldn’t be an “authentic” buttermilk. (Buttermilk falls under the same labeling rules as regular milk, despite the fact that regular milk fresh from the cow averages around 3.5% fat or so [varies by cow species], while naturally produced buttermilk averages much less fat, because so much of the milk’s fat has gone into the butter.)
BTW, if you don't want to throw out the butter as a by-product in your buttermilk production, it will help to knead the soft butter for a while to smooth it out more. Also an opportunity to add herbs and/or spices for a compound butter - but you might not want to cover the purity of your fresh butter's flavor.
2. Fluid dairy products are almost infinitely malleable. Want more fat in your buttermilk? Add cream. Want more viscosity? Add cream, sour cream, or non-fat dry milk powder (aka “instant milk”). Want it tangier? Add a little yogurt and let it sit at around 105°. Or just concentrate it by adding the powdered stuff.
But to address your comments on commercial products, and those of one manufacturer in particular … I know when I was Marketing Director at Dean Foods throughout most of the 1980s, we thought most of our plants made damn fine – and way underappreciated – buttermilk – especially at the Chemung plant (just west of Harvard IL) – which is where most of the Chicago-area product comes from. [I can’t speak to any changes that may have been made since the Texans took over, but I’d be surprised if they changed much of the buttermilk equipment at the Chemung plant.] Most commercial buttermilks these days are pretty good, and reasonably close to pre-WWII buttermilk. More importantly, virtually all recipes calling for buttermilk developed since WWII were developed using cultured lowfat buttermilk, since that’s the only stuff supermarkets commonly stock.
Just my 2¢ worth.