Boot Mac OS and mount the EFI partition of each drive with EFI Mounter.
If there is nothing in the partition then Win10 had nothing to do wit it. Pilgrim is correct that if you use Mac OS Disk Utility to format a drive GUID Mac HFS+ then it automatically creates the EFI partition.
Check your system definition - what you describe happens when you try to install with a system definition of an older Mac that the new OS does not support. i.e. iMac 13,2 will run anything up to High Sierra, but will not run Mojave or Catalina and will give you the slashed circle.
Bootcamp does not work with non-Mac hardware. Disconnect the Mac OS drive, connect drive for Windows, install Windows, shutdown, reconnect Mac OS drive, boot to BIOS/UEFI and set BBS boot order to boot Mac OS drive first - save&exit, continue boot.
What is the speed throughput of the WiFI card? If you are going to connect Wifi, how many other devices are you connecting to your WiFi interface at the same time? This is what makes the difference in the connectivity speed to the web you need.
If you are having problems with an update it is probably because there is a firmware update included in the package and, since you do not have Mac firmware, the update fails. Something like this:
So, just run the SecUpd files for the OS and Recovery and then delete the folder. I...
I would suggest a NAS for your storage - especially if you have a large library of audio effects. Other than that, minimum of 16GB of RAM (32GB would be better) and a 8-core or better CPU. Boards are pretty much the same between brands for price and quality - you just need to find one that has...
Actual assembly of a computer is quite simple if you can follow written directions. If you do not want to do the actual assembly yourself, there are many custom build shops in the UK like Computer Planet or OverclockersUK that will assist you in selecting your parts and assemble them for you for...
It should not matter one way or the other as long as you have an EFI partition large enough to satisfy both installers. Suggest at least format/partition with the Mac Disk Utility first, then install in order desired.
Did you add Lilu and Whatevergreen to Clover/Kexts/Other?
The attached are the kexts and the drivers I use for High Sierra. You also need the Clover patch in config.plist to allow installation on a "dropped" Mac Pro that Apple no longer supports if you use the MacPro5,1 or earlier system definition.
Try it with Mac/AROS on one drive and share the Win10 with Linux. With Ubuntu that is fairly easy, just select "Install beside Windows" at the install screen and the Ubuntu installer will take it from there. Thinking about it, this is probably your optimum distribution of the OS's.