This is standard corporate technology and needs standard ordinary technology. There is zero requirement for this sort of application to use anything fancy. Indeed if you use fancy technology that scratches the itch of a tech bro then you are buying yourself problems hiring and recruiting developers - this is one of the worst problems you can have. Not such an issue right now while the employment market is down, but a massive issue when it is not.
Use ordinary technologies, any of these things are fine:
Java
C# .NET
Python
TypeScript
Postgres
Also, don't use cloud specific technologies such as serverless. Just write software that can run on an ordinary Linux server. Once you start using serverless you are bound to that code and that cloud forever. Also, and other may argue against this, but my experience of building application using cloud technologies is that way too much developer time and effort goes into making the cloud work and not enough into writing application code. Have a general directive in place to use the cloud for running compute instances and avoid using cloud application services unless really necessary and have a process where doing so requires signoff from the top. Nothing wrong with cloud virtual machines and email sending but just run most other things on Linux - own your own computing infrastructure and be in a position to pick it up and move it elsewhere or even on premises if you choose.
In summary, beware tech bros advocating anything except Java/C#/Python/TypeScript/nodejs/Postgres, don't use serverless and use cloud computing for virtual machines, email and DNS and not much else.
Hire a development manager with experience building corporate applications and let them hire three developers, a tester and a business requirements analyst and that's phase one of your "bring it in house project". Build a series of small chunks of functionality, don't bite of more than you can chew.