pH 8.2 should be fine for nearly all common rift lake African cichlids. With the exception of some river species or some of the smaller or lesser known lakes, it's similar to their native pH. Actual mineral content varies between the lakes; for example Tanganyika is higher in magnesium than Malawi, but an exact match to native water isn't necessary and this is true of most fish we keep.
50% water change is not excessive at all under normal circumstances. The notion that you can never change too much water is generally true, but there are exceptions and other considerations, some of them depending on what's in your tap water. You can have perfect local water where the (generally) impractical ideal would be to have an open system-- clean water flowing through the tank on a constant basis. But you can also have problematic local water, better served by allowing your tank ecology to stabilize, or even clean, your tap water, and not 'overdoing' water changes. I've had both excellent and problematic local water. There's no one size fits all rule here.
Then there's parameters. If your local water suits your fish with no adjustments, that's one thing. If you need to adjust your water for your fish-- and, despite the horror some have of managing water parameters, It IS NOT A SIN to do so and can easily be managed in many cases, just a matter of knowing what you're doing, or a bit of tinkering to arrive at a reasonable routine. I've done it for 20 years in my current residence to make my 6.0-6.6 pH well water fit for frontosa and other Africans, including adding a bit of magnesium for kapampa gibberosa. In Florida my water was perfect as is for a variety of fish, in New England I lowered pH for blackwater tanks. In Chicago as a kid... well, I wasn't sophisticated enough about water chemistry to know much different.
...The point of the above being if you do adjust your water to suit your fish, do you really want to do this every day, every second day, or several times per week if it's not necessary?
And that's my larger point. How much water you
can change is one thing. How much you
need to change is something else entirely, and this varies between different setups, stock lists, and locales. Do some reading in planted tank communities and you'll see some of them change little water and/or at lengthy intervals, lot of plants, not many fish, expertise with water flow, filtration, nutrient dosing for the plants, etc. allow them to do this-- don't try this with your Oscar, or a tank crowded with haps and peacocks...
The paradigm of immense water volume or flow doesn't apply to all natural aquatic ecosystems and fish species, including the Amazon region-- there are ponds, oxbow lakes, even ditches that don't receive weekly 'water changes'. None of this is against water changes, high volume water changes, or frequent water changes-- Again, I'm saying one size doesn't fit all.