How to design the joint work of several applications of the same organization?

The problem is as follows. The company has several self-written programs, more details below:

1. one database MS SQL;
2. client machines (Win8.1 / Win10) have a Windows Forms client installed (not much functionality, about 20-30 noncomplicated forms), which interacts via WCF with the database (base from point 1), authorization and user rights check using LDAP (the company uses Active Directory);
3. site on Laravel to perform some tasks with data from the database, site users are remote, temporary employees. The business processes of the site on Laravel and WCF services are sometimes duplicated, the base of paragraph 1;
4. there are still services that perform background tasks, for example: sending email from a formed queue;
5. other external applications with their own databases for training and certification of personnel, as well as the organization's website on CMS Wordpress (hosted on external hosting).

At the moment there is a task to add and update the current functionality, as well as to bring all the sites/applications to the same style (so that the design matches the brand). So I think, to implement all this in C#, so that users and admin do not suffer from the synchronization of data between their own and external applications. But what technology to choose for each client? In addition, need to create a mobile application.

I have my own thoughts on architecture, but I would like to first hear the advice of experienced people. Thank.