Leave things as they are
Your Linux systems continue to be isolated and harder than necessary to manage. That may or may not be an option for you.
Buy from somewhere else
There are surely other companies who could provide you with this service. After you've found somebody you're unlikely to see a fixed price, money-back guarantee or a streamlined delivery process that takes only a week in total. Contract negotiations tend to take 1,5-2 weeks based on our experience, as opposed to reading our terms of service which takes about 5 minutes.
Do it yourself
If you wish to pursue this route we recommend following Red Hat's official documentation on direct Active Directory integration using sssd. Then just repeat the process on every Linux system you wish to integrate.
We do not recommend doing indirect FreeIPA integration yourself, unless you're willing to invest lots of time in it. FreeIPA is great, but like all complex systems there's a steep learning curve.