If it wants to sit at 7.8 that's not a problem. Leave it there. No reason to fight it.
Reassuring to hear the views here about leaving it at the high end. My intuition would agree, since the pool seems absolutely great as it is, no problems at all. It'll certainly help on the usage of MA to not fight it back down to 7.5 all the time.
I understand the color changes can be tough to estimate. The numbers are recommendations not be all end all, if your pool likes to be at 7.8, and you aren't having issues with scaling, water quality looks good, no issues, then why not run it at 7.8.
It's important to know why you're running at 7.8 (the high end of the scale). If you let it run to 8.0 and knock it down with 7.8 and you're not having scaling or any other water quality issues then I say go for it.
Can you post a full set of test numbers? There may be some advanced discussion issues that may be able to address the scaling issues.
What's the TA and CH levels? What about your fill water?
Adjust the automation so the spa only runs 1/2 hour a day instead of all day long. That should exchange enough water so the spa stays chlorinated.
Shut the valve off that returns the water to the spa. There are valves that return the water to the pool and the spa and valves that control the suction from the pool and the spa.
Yes, SWG's can cause the pH to rise but using our guidelines it can be limited or eliminated all together.
You need to figure out how to program the valves to operate the pool the way you want to control it. The way it is now with the water returning to the spa you will always have fast pH rise.