If you only use K2CO3, KH will still fluctuate. It's best to use mainly CaCO3, MgSO4 and a bit of K2CO3, all together. And if you add these as separate powders, then you will need to measure it carefully to achieve a 4:1 Ca:Mg ratio. Or follow my DIY mix formula that Zoidburg linked earlier.
Just note that I used CaSO4 in my formula, as my aim there was to not alter KH. Replace with CaCO3, if you want to raise KH. It essentially becomes similar to the Salty Shrimp GH/KH+ product.
This is my recommended method.
However, if all you have or can find is K2CO3, then add this at every water change weekly (or twice weekly). This is much more useful than NaHCO3.
Avoid bicarb soda. It adds nothing to the tank using it. You are better off using K2CO3, at least it is a macro fert. All it will do is send your KH and pH sky rocketing. The fluctuating pH will kill your shrimps.
Potassium sulphate will raise GH only. You can add more to raise GH. Potassium is very safe in a tank, and is very difficult to overdose.
Just remember, if you raise KH, you will also be altering pH. So you want to do this slowly over several days, maybe even weeks.
Neocaridina shrimps don't really need GH 8 and KH 4. They will be fine at GH 6 and KH 3.