Clean SQL database from whitespace, remove duplicate entries and check for fragmentation speed up database

We have today a financial application using SQL express 2019, the sql DB is today approx 1,6 GB when dumped to backup. It takes today roughly 17-20 sec before the database is loaded into to application and the data is visible. The company that has created to program doesnt seem to have knowledge of how to clean up the database and optimze.
The database has been upgraded and upgraded since 2008 and there is no way to rebuild the databse unless
we start with an empty database and add all the customers in manually into to the program again. Which is not an option cause it would take very long time to do.
We are now looking for someone that has SQL DB knowledge and are able to script the DB to clean it from duplicate entries, remove fragmentation and speed up the database.
We will be able to provid a virtual server with the database and applications for testing and for the person to work on, you would able to see how the database work with the program and what can be done.
A "Live" VM, no different from what we use in production.

Microsoft Server 2019
Microsoft SQL Express 2019
Wolters Kluwer Bureau