First and foremost: salinity. Get that dialed in by using Randy's DIY calibration solution. I feel like 75% of the paramenter issue threads we see are just simply bad salinity
Second, temperature. This doesn't need to be ketpt super tight but you need a reliable way to monitor it
Third, alkalinity and calcium Salifert is my go to here. Cheap, easy to get, and easy to use IMO
Fourth, nitrate and phosphate. Salifert for nitrate and Hanna for phosphate. These two things will give a good idea of your import/export methods aka filtration.