How to debug bootloop?

I created a custom Cyanogenmod 13.0 for the Galaxy S5 Exynos (G900H) , however after I fired up my ROM the phone got stuck on bootloop when the Samsung logo turned off and in the loop.

since CM and LineageOS do not support my particular S5 model (G900H) - I added missing folders (device, kernel, hardware, etc.) from the fevax repos ( https://github.com/Fevax ).

After fixing some compilation errors and adding all the required fevax repositories, I successfully built the CM ROM zip file using the "brunch k3gxx" command.

However, as I said, flashing (after factorty reset) using TWRP recovery goes to bootloop. I have also tried flashing only restore / boot / system partitions with heimdall, but also results in boot.

I thought that maybe the problem was in the toolchain, but after a lot of research it turned out that this is probably ok (prebuilts / gcc / linux-x86 / arm / arm-eabi-4.8 / bin / arm-eabi-) ...

Also, fevax posted my own ROM for this exact model and I rolled it successfully and it works great. I tried to contact him (email, xda messages, etc.) but he hasn't answered me yet. I want to create my own ROM, so its ROM only proves that it is possible to create a custom CM-ROM for this special S5 model.

Currently I have no idea how to figure out what is causing the bootloop, the "brunch k3gxx" log shows no unusual warnings or errors.

So, is there a way to debug bootloop - is this Samsung Galaxy S5? Or any other way to get information on why this bootloop exists?