You're just using the wrong part. If you get an STM32 or NXP iMX, there are tens of thousands of pages of TRM available to describe the function of every block inside the SoC. The core iMX7 technical document is over 7,000 pages.
You won't get everything, especially anything ARM has licensed out to the chipmakers like the CPU/NVIC/AHB, but it's a lot deeper than you probably need.
i.MX isn't perfect, like there are still closed-source microcode blobs for things like SPI DMA, but it's better than nothing. You can at least build it into a design without having to worry that the supply of boards is being strangled.
You won't get everything, especially anything ARM has licensed out to the chipmakers like the CPU/NVIC/AHB, but it's a lot deeper than you probably need.