Is there a C/C++ SDK for Raspberry Pi boards (not Pi Pico)

Question

I am currently working on porting ThreadX to the Raspberry Pi 3 (or newer). After examining similar projects undertaken for the Pi Pico and Beaglebone, I realized that I would require the following:

1. Processor-specific source files from ThreadX, which are available for the Cortex A-53 architecture used in the Pi 3.
2. A Software Development Kit (SDK) that facilitates bare-metal programming in C/C++ and provides all the essential startup files, drivers, interrupt handlers, etc.

While I am aware of the existence of an official SDK for the Pi Pico, I have been unable to find a similar SDK for the Pi 3/4/5. Consequently, I have the following questions:

1. Are there any third-party SDKs available that I could utilize?
2. Is it possible to integrate ThreadX without a proper SDK?
3. I have examined the work done by RasPi-Tutorial and David Welch on Baremetal, and I have been successful in running basic C programs without an OS. However, I believe that I will require more extensive hardware abstraction than basic LED blinking and UART communication for integrating ThreadX. Any suggestions if I choose to pursue this path?