I'd say to give Kalkwasser a chance and see if it keeps up with your tanks Alk & Calcium needs. If it doesn't, you can then explore the other options like 2 part or a calcium reactor.
If you decide to go with adding Kalkwasser to your top off water, you might want to consider adding a piece of hardware like the Tunze 5074. Kalkwasser infused top off water significantly shortens the life of any pump moving it out of the storage tank. You can put something like the Tunze 5074 inline between the RO top off and the sump and extend the life of your top off pump. It's basically just a container that you fill with a Kalkwasser paste and the top off water moves through it on it's way to the sump.