Week 4 DWC update, EC climbing issue

Running a Gorilla Glue pheno right now, day 28 of veg in a 4x4 with a 600w HPS and a supplemental Samsung QB for side lighting. Reservoir is sitting at 68F, pH locked at 5.85, and i've been targeting 1050 ppm (roughly 2.1 EC) for this stage. Plants look healthy overall, stacking nicely, root mass is white and dense.

The issue I ran into: EC has been creeping up on me between reservoir changes. I top off with plain RO water to keep the level, but by day 3 or 4 the EC is climbing toward 2.6-2.7 even though I'm only adding water. My tent sits around 77F with lights on and VPD is around 1.1 kPa, so transpiration is reasonable. My read on it is the plants are drinking water faster than they're consuming nutrients, which is concentrating the solution.

I dropped the mixed nutrient concentration on my last full res change to bring the target down to 950 ppm, and i've been topping off more frequently, basically every 36 hours instead of every 60. That seems to have stabilized things around 1.0-1.1 EC between changes, which feels a lot more dialed in.

Curious if anyone else has dealt with concentration creep mid-veg and whether adjusting the base ppm lower was the right call, or if there's a more systematic fix. Also open to feedback on whether 68F reservoir temp leaves enough headroom before I start seeing issues.