I have been meaning to build a 4-8 channel mixer for recording drums/guitar (multiple mics) and if that is your purpose, then buying an off the shelf 'product' is cheaper and of course easier. However! As you wish to learn, then you can break it into a modular system, after all, once you've designed one channel.. you just add more.
Before getting custom PCB's though, you'll have to do a fair bit of bread boarding to find test out each part, and of course, learning along the way. From a solderless breadboard (which will be unshielded = noise) to perhaps a veroboard prototype, again just one channel. Getting 10 PCB's is probably about the same cost as getting one or two, so you'll have some spare. I would advise, if space allows, adding extra footprints for parts that have different packages, lots of test points, and perhaps even trimmers if you wanted very accurate gain/EQ centres (probably not worth it though). Not trying to make life more difficult but I always thought a latching 'mute' button would be handy as well.
The backplane can have the PSU, master volume, VU meter (with clipping indicator) etc.. At least that is how I would do it.