P.S.
For anyone else doing such a big upgrade jump you need to checkout the setChroot method. My classes are outside webspace, but the resources for the PDFs, especially any images such as logos, stored in web space and referenced relative to the php page being converted. e.g. <img src="../resources/myLogo.png"> I added the following to get images to start appearing in the PDF again.

$dompdf->getOptions()->setChroot([dirname(__DIR__)]);

A little experimentation will fix any issues.