

Those co2 cartridges are like $30 for 20 16-gram charges. I have them for my ukeg (pressurized growler). They keep 2 gallons of beer pressurized and dispense it, but I’d assume they are only enough for about 2 liter to carbonate from still. So roughly 10 gallons for $30 vs 25-50 gallons for about the same price (only considering ongoing CO2 costs) with the keg setup.
So you can, yeah, but it’s substantially more expensive in the long run, and a 5 gallon keg plus everything else really doesn’t take up a lot of space. They are tall and narrow, and you need to put the soda syphon thing somewhere, too, and I assume that’s taking precious counter space, rather than sitting in a dead space corner behind a door, or tucked into a closet or something.
Really it’s just a matter of how much you’ll use. If you don’t go through 5 gallons of carbonated water in a reasonable timeframe, like monthly so you can clean everything out, it doesn’t really pay to make a big batch. You can get a 2.5 gallon keg if you want a smaller batch, but those are tough to source used and are thus pricier (place I’ve bought from before has them for $120, about the same price as a full-size keg of the same brand)
The way we made it at the place I worked was to puree fruit, then reduce it a bit over heat (no added sugar), then put like 2tbsp or so over ice and topped off with the carbonated water.
So it’ll only really work well with flavors that don’t degrade in heat, but you may also have luck with just straight pureed fruits like watermelon, not super sure how that’d turn out.
You can also use fruit syrups that you buy or make yourself, tons of recipes out there.