Hello, I am an environmental engineer sharing my design for a ~300 gallon IBC tote rainwater collection system. What makes this different and beneficial for self-sufficiency is the low cost per gallon. This tote also provides a large enough storage tank to use a dedicated irrigation pump for reuse. You could make this as simple or as complex as you wanted to.
IBC totes are frequently used for storing and transporting food grade materials, so there is an aftermarket for cheap used totes, you just have to verify what they were used for. I got this 275 gallon tote off of marketplace for $100 USD. It was previously used to transport food dye. They are highly durable because they’re built out of HDPE and galvanized steel.
Other Materials:
Leaf filter, from $10 to $70 - my preference is the Leaf Eater Advanced ($66)
Downspout diverter, my preference is Earth minded Flexifit diverter ($20)
PVC pipe and valves, figure $50
Total entry cost including tank: $180 - $240
Optional:
Lid adapter for two inlet ports $10
1 HP irrigation pump $140
Discharge splitter $10
Wi-Fi controller $30
Piggy-back Level float, alternative controller $50
Ozone injector to keep the water fresh and odor free: $400 (luxury) JED Engineering model 203
To be fair excess fats are bad for the normal drain too. Temperature is a good point, my initial idea was that the channels would be along the grass paths so if a bit of grass dies by the opening it isn’t really a massive concern. By the time it soaks in any real distance it should be cooled by the surrounding soil.
Bath/shower usually is the cleanest source to go for first, the pipes are next to each other for me. Either way I would set it up so that they can be easily removed when I don’t need the water going onto the garden.
Do need to check how flat the area is, exit point from my house is pretty close to the ground so a slight elevation change could be a problem.