Test the CH of your fill (tap) water. If it's high, then that is your source of high CH. Evaporation and refill will only increase the CH over time. It is lowered by water dilution if the CH of the fill water is lower than that of the pool water. It can also be lowered by reverse osmosis (RO) though companies using that process may not be available in your area.
To deal with high CH you can target a significantly lower TA level and a lower pH, but it will be hard to keep the pH lower unless you lower the TA. The use of 50 ppm Borates in the pool can help keep the pH from rising as quickly, if that has been happening. There are also metal sequestrant products that primarily sequester calcium (they're called scale inhibitors), but that would be a last resort if you've tried everything else.
Can you please post a full set of test numbers -- pH, TA, CH, CYA, Temp and whether this is a salt pool and whether you use borates? From
this post you listed "TH" implying Total Hardness, but the TF-100 doesn't test for that so was that a typo? You also noted 400 ppm, but now you are saying 750-800 so I'm confused. Your TA (120) and pH (7.8-8.0) readings were high. You can use
The Pool Calculator to calculate the saturation index. If I use the 400 ppm CH reading plus your other readings (assuming 7.8 for pH from your range) and assume 70ºF water temperature, I get +0.42 for the saturation index. If I use 800 ppm CH and 8.0 for pH, I get +0.84 for the saturation index. This is why you are seeing scaling -- the light brown tan stain on the walls and floor of the pool as well as white crust on the tiles.
If you were to lower your TA level to 70 ppm and get your pH target down to 7.7 then the saturation index would be in balance at +0.04. However, you'll want to remove the scale you already have so to do that you'll want to significantly lower your pH (making the saturation index negative, at least temporarily to remove scale), but if that scale redissolves it will increase your CH level even further.