Quick few questions... as I have experienced a similar problem and have been actively working solutions myself in a similar setup that you have described -- 2 part, fuge, CO2 scrubber
1. When your pH gets to 8.3 or it's highest point is that related to when you change out the CO2 media to fresh media? Or strictly just the water change?
2. How big is your water change? Any additives into the fresh saltwater?
3. When pH drops, is it dropping over hours (see screen shot below) or slowly over days?
4. How big is your CO2 media reactor? How often do you change the media?
5. Does your air intake go to your CO2 media reactor? Or are you feeding outside air separately into the tank?
My initial thought is that you are burning through media quickly (solely based on my own experience). I would get anywhere between 48 to 72 hours (2 or 3 days) using the BRS 10" media canister. I've since made a much bigger reactor and now can get a very consistent 8.24 to 8.28 variance between day/night over a couple weeks. The chart below is when my media is depleted and crashes fast. I have a pH alarm at 8.15 to warn me I need to change my media.
Water change containers sound like they are far away from a heavy CO2 area, which probably explains the higher pH but can see why that would be only a temporarily affect. CO2 absorption varies by setup and location but in my house you can see a very rapid decline in pH which I've been able to attribute entirely to CO2. For example, the chart below is my media getting close to end of life + an additional person in my house during that time period. Starts to increase as my guest left.
I haven't seen any benefit of running outside air into a CO2 media scrubber other than it may extend the media life slightly longer. I've dabbled with a CO2 re-circulating setup but find that the moisture accumulation causes a different set of problems that require additional equipment (such as solenoids) to manage.
There are options such as dosing Kalk, not via ATO but via a dosing pump to manage consistency and amounts, to me this can create a precipitate problem but is manageable if done correctly and slowly. If you are able to achieve a higher pH value already, changing your dosing regiment may cause different problems.
What ever you do, do it slowly my friend. If this hobby has taught me anything, it's patience. LOL!
For comparison sakes. I run my dKH around 9, Ca around 440 ppm and Mg around 1300 ppm. In order to achieve that I need to dose 215ml daily of Soda Ash and Cal. I dose about 40ml daily of Mg.