How does CiviCRM rate compared to other CRMs
I've worked with sites with tens of thousands of records that are constantly being interacted with, changed, etc. So that is no problem.

There is some tracking of changes. What exactly were you looking for?

Whether it can integrate depends on your systems. We'd need more info on what you need to integrate with. Otherwise the answer is maybe.

I've found it to be user friendly, but I've used it for a while. You will likely need a bit of training up front to help you locate everything you need to do. I find that much easier than trying to learn it yourself and much less time consuming as well.

The same thing with customizable. Yes, it is customizable, but it depends on what you want to do. Some customizations may mean a major edit to the software, that an extension needs to be done, etc.

For support, it sounds like what you're talking about is someone you would hire to support the install. There is some support on here, via asking questions and such. But really getting into your site and fixing things, changing things, running updates, etc is going to mean the need to hire someone who knows Civi.

Some of the security relies on you and your server, how often you run updates to the CiviCRM software, etc. There's also user logins that have specific permissions as to what they're allowed to do.