Would it be possible to individually select the ASR module and only copy that?

No, it's not a module on it's own (ASR=ASRock, they use this for many things within the BIOS). The "Secure Erase" module that is same in all BIOS, only differs by two bytes in the 40B vs 51A module, so it's possible this is the ASR module, and those two bytes enable it.
But the setup and AMITSE must also be copied over even if that two byte change alone enables the module to function, because the Setup/AMITSE modules contain what you actually see and all the functionality within the BIOS, so they must be swapped as well.

Would I be still able to overclock all 8 cores 16 threads? Would things like DSDT and CPU SSDT and CPU microcode be changed?

Yes, but you wont see the individual settings options past certain number of cores in the BIOS. No, all that rest you asked about would be same still.

Are the 2, 7.51A Secure Erase BIOS (Table Edit and GUID Swap) edited for capsule for secure flash? I can't find them on UEFITool. I'm going to try both soon.

@MrJack - Please zip up both BIOS you are asking about and upload to some file host so I can check, I don't remember and I have too many BIOS files here in a folder for you to be sure which you are asking about
I will check, since I do not remember, and fix them for secure flash if needed.

Here you go.

Thanks, yes those look to have capsule removed already, so should flash properly. If not, then they have Asrock's new protection which has as of now only been found in 300-series so far
If they give secure flash error, you will need to flash using this method - [GUIDE] Grub Fix Intel FPT Error 368 - BIOS Lock Asus/Other Mod BIOS Flash

If I were to upload the ATX BIOS and newer mATX BIOS would you be able to update the ATX BIOS using the mATX BIOS?

I just thought, wouldn't it be easier if using the stock BIOS, microcode for 8-core was inserted? What else would have to be done?

Thanks for posting in a thread, at least now I know some of what you're talking about Do you mean you now want same edit "Secure Erase" from some mATX BIOS to an ATX BIOS? That is possible, but will mess things up as mentioned, due to the differences in board layout etc.

Microcode for 8 core? What else has to be done, for what? If you mean for some coffee lake upgrade, LOTS has to be done, microcode is the least thing that has to be done there (ME change, vBIOS edit and change, GOP change, ACPI Tables edit, power modules edit, microcode change and maybe more I forget).
There is now a tool for this kind of upgrade, have you seen it's thread - [TOOL] Easy automated Mod tool for Coffee Lake bios

Here we go:

This 7.40B, Asrock sent me this with Secure Erase added. ME 11.6.0.1126 I think.

Id like Priority: 1.7700k 2.9900k and if you can 3.6700k.

My aim is to use this BIOS so I can keep Secure Erase. This is all so bloody complicated :/

Do I still have to do other things? like power and all that other stuff?

Yes, you will need to do all other things mentioned, that is why I suggested you use the AIO tools I linked you to, it's much easier and quicker.
I will edit in BIOS mod shortly.

How often do you need to Secure Erase? Why not just use the CPU compatible Coffee ready BIOS now?
You can secure erase with other tools any time, you do not have to use some built in BIOS tool.
See also, this, it's fake Secure Erase anyway! >>
Why in the hell would "Simulated" SE even be an optional setting choice in anyone's BIOS!!
I can change to real, but still, that's another good reason to just use other SE methods.

the one asrock added is ASR

ASR Secure Erase Tool. I tested it, it works.