Go with the DOS for water changes and the BRS dosing pumps for Alk / Cal / Mg.
As for being gradual, mine changes 250ml/min. It's programmed to do 725ml water changes 12 hours apart. You can make it as quick or as gradual as you see fit.
A picture of the interface might help. It's completely configurable, so you can do whatever you want.
Everything is plumbed into 2 x 65g storage tanks in the basement. About once every month, I get an text from the Apex saying it's time to mix salt water. A few minutes and 3 valve turns later, the salt tank is re-filled and then mixes for 24 hours. During that time the water changes are suspended.
Once the salinity is checked, the water change schedule is turned back on and I go for another month.
The second mixing tank is RO/DI for my ATO.
Once you go to an Automatic Water Change system, you'll never go back. If I only had room for a Theiling roller mat.......