Tbh I usually dont let it sit long so the fact that it isnt as carbonated is less of an issue. That said, there is also fermented soda which uses wine yeast (you can use baking yeast but this is arguably less ideal) and sugar to generate the CO2 instead of a bicarbonate base and acid. This you can essentially let sit to carbonate as much as you want but like all common ferments, it does generate a small amount of alcohol and does require dechlorination of the water you use. It also needs sugar to work while soda stream and the soda/acid method dont care how sugary the soda is.