Good to her that the new shrimps are doing well!
I would bite the bullet and make your life so much easier and buy the GH/KH+ shrimp product, that with RO water will produce the right water in ALL parameters, as long as there isn't anything in the tank such as rock that alters it out of those preset parameters. The new water you add will be the perfect balance of GH/KH/PH/TDS and you only need to prepare it using TDS meter (make it to TDS 160 etc) so it is very easy and straightforward. As long as the new shrimp seem fine I would just start using this new water from now on so it will very slowly make any adjustments, so shouldn't harm the shrimp - I wouldn't do any large water change if all seems fine for now?
As JayC mentioned, if you are using baking soda it probably isn't the right product?
It may even end up costing you less as well as making life easier in the longer term.
Simon