How to keep loaded entities in memory in .net and ef

So, I am working on a legacy application written in .NET 2.0 and C# that's only purpose is to execute workflows (background jobs or processes). In essence what it does is:

1. On app startup it loads all objects from DB into memory (in classes called Holder)
2. Start the processes (multiple of them) each in its own thread
3. These processes works on the data in memory (multi-threaded) so the data access is kept between locks.

So the above picture depicts the process to work with the data.

I have the task to modernize this using new .NET 7 + Entity Framework and its capabilities. Because this application is heavily multi-threaded and many of the infrastructure classes like (workers, holders, dal access) are singletons there is locking everywhere.

My wishes are this:

1. Because of multiple threads accessing the same data, I would like to get rid of those singletons. But the problem for me is where should I keep the in memory data the workflow needs to work with it.?
2. If I choose to not remove singletons, then I was thinking to use Repository pattern to hold the entities in memory, but then I they should be singletons any I don't know if this is a good idea.

So my problem is where to keep the data once is loaded so that many threads can use (read, update delete) it without going into DB each time?

I need a design pattern for this, or maybe use some sort of Level2 cache for Entity Framework