So, the major part of your current system is serving your business and meets your requirements, GDPR and MTD isn’t a good enough reason to throw it all away. It’s like throwing out the Baby with the bath water.
You know your system so you know what you need to do!
I am guessing your problem is probably due to lack of resources to work on your legacy applications. Your IT team probably all want to work on the new stuff, they don’t want to be tied down to the legacy “old fashion system”. The new technology is where all the exciting work is but it’s the legacy system that has got your business where it is now, its trusted and its reliable and has probably served you business for 20 years or more.
There are companies out there like us who can build value added add-ons to your solution to meet these requirements. By doing this It’s less risk to your business and a short-term cost.