This is indeed a big project.

If you do do it yourself, you should do it in small stages. From your post it wasn't clear how many rooms had surround sound (all of them?). If that's the case, you might be better off running optical to each room. If not, then do the smaller rooms (the bathrooms?) first as these will require mono sound instead of stereo.

The key with this setup is documentation. Did the previous installer leave you diagrams so that there's a way to know what runs where inside the walls? If not, you'll have to spend some time testing with a voltmeter and labeling everything. You'll need to calculate impedance loads for each room too. Your amplifiers will need to be able to drive these loads.

Another big question to ask is if this will be for heigher-end sound or just for background music. Maybe you might want the hi-end in some rooms but not all. Of course, if all speakers are in-wall, then it won't ever be as good as box speakers.

As this is a big project, there's also a myriad of options and suggestions other can give you (for example, how to control volume & source). Doing the project in stages and will allow you to post your questions here as they come up at each stage.