How to encrypt or store data so that only a pool of processes can share it on Windows?

I am writing an application T1 which is using SQLite database to store some sensitive data and i am trying to encrypt it securely. The issue i am facing is that i don't want my symmetric key to be stored in memory as in that case any other process can take a snapshot of memory and access my keys. I have looked at many possible similar questions:

CryptProtectMemory/CryptProtectdata

But the issue with this is that it has two different options -

CRYPTPROTECTMEMORY_SAME_PROCESS or CRYPTPROTECTMEMORY_CROSS_PROCESS

The problem with CRYPTPROTECTMEMORY_SAME_PROCESS is that it restricts the access to a particular process (and my app has a seperate UI process which could also access the database) and the problem with CRYPTPROTECTMEMORY_CROSS_PROCESS is that it allows all the processes of the logged in user to access this data which kind of defeats the purpose

So what I want is an API where i can specify only a pool of processes that can access my data/key

I looked at similar options but nothing is applicable in my case:

1. Using DPAPI - This ties the encryption key to your admin login but the problem is that i don't trust the currently logged user. In fact, I want to hide this from anyone but T1 and my UI process.I understand that i can add secondary entropy, to restrict the currently logged user from accessing the data.However, I need to store this secret data on the machine. How do I protect that... Appears to be recursive problem.
2. External hardware or storing the key at a remote server - As the app will be deployed at many commercial endpoints(which may not have the specific hardware) and it needs to work in offline mode also(the server may not be accessible).
3. Store the key in the database - I need to secure the database, which is kind of recursive in my case again.