The first (and frankly, only) rule is to keep it simple.
Don't worry about being "harsh" on leather. 99.9999% of car leather is coated. Thus, it isn't anywhere near as delicate as furniture leather. "But mine is different..." NO, it isn't. Trust me on this. I have detailed high end cars with leather seats using Meguiars All Purpose Cleaner and it works great. Dilute it around 10:1 and you'll be set. (Mothers makes a similar product with a slightly less odor). Most AP cleaners might also do the job but I tend to stick to ones made by car care companies. The problem with "leather cleaners" is they are almost always too weak to actually clean. I used Chem Guys Leather Cleaner, a popular choice, followed by Megs AP. It was like I hadn't even cleaned the seat after the CG product -- the AP picked up a lot of missed dirt. (O'Reillys sells the Megs products -- you have to ask for them and order them for 1/2 day in store pickup. Superior Products makes similar cleaners, also sold at O'Reillys).
On the dash, I tend to stay with a lighter product as the dash mainly gets dusty rather than dirty. With a wet/damp microfiber rag, use some Griots Interior Detailer. For doors, steering wheels, etc., the AP should work fine. If something is REALLY dirty or has been neglected, steam is your best bet. You don't need a pro steamer; a decent consumer model works.
For really dirty carpets, I'm sad to say the only way to get those really clean is a good steamer and a carpet extractor you'll need to rent. For lighter duty, I use something like Megs Super Degreaser (~8:1), scrub the carpet, steam it, and pull up with a consumer extractor or vac. This will work fine in many/most cases.
Hope this helps and at least gives you some ideas. Your interior stands up to normal abuse when people are in it; cleaning it with good products is, at most, inflicting maybe 25% of that abuse -- but to its benefit. Most people don't think of it that way and wonder why their car is still dirty.
