Would a sodastream type solution satisfy your needs? Things like that have gotten super cheap. My dad also just uses unsweetened drink flavoring in his like bubly. Can also use natural flavours instead of buying artificial ones. Concentrated lemon juice is easy enough to find, I'm sure other concentrated flavours are possible to find or make too. But yeah, by default it just makes carbonated water, adding flavour or sweetening it is an optional second step.

Devices that carbonate water have been around for decades, the upside of the new ones is just that they hit a critical mass in enough markets now that the co2 cartridge replacements are everywhere and cheap now. I live in a small town of 2000 people up in Canada, and we have a sodastream canister exchange program in our local hardware store, lol. And a canister is 20$cad and lasts me about 100 drinks, the water I use is filtered tap water, so the price there is pretty negligible per drink. Totals up to about 25 cents per drink. The price of the machine itself was 80$cad, so if I only add that to the first batch, those first hundred liters were more like a dollar each. Pretty easy to swallow. Though it's carbonated, so best to sip anyway.