A couple of things to bear in mind here,
1) Your talking about over 75 years worth of comics, usually at least 2 a month (Detective and Batman), that is probably waaaay more comics than you realise
2) of those 75 years of publishing it goes without saying that even Batman went through some pretty rough periods (it was even going to be canceled at one point in the late 60s)
3) The writing style in the first 30-40 years of comics is so very different to anything thats out now that if your not already familiar with it it can be very jarring and unpleasent for some
4) Not all of it has actually been reprinted, and some of it (a lot of the silver age stuff) is only available in black and white still.
If you still want to start from the very begining then DC are splitting the big Golden Age Omnibus into multiple smaller paperback volumes, the first of which can be found here
https://www.amazon.co.uk/Batman-Gold...dp/140126333X/