New system build, new long boot times

If you really want to stick with RAID setup, then check your bios settings. All that is related to hardware.

I'm sure, that you checked internet for similar problems (and there are plenty) for you Motherboard. I would like to point out two of them:

DRCT Direct key should be set to disabled, if you don't have appropriate case. Discussed here.

and one other suggestion:

Kerry Neighbour Alix Niculae • a month ago

I had a similar issue - except my time was almost 3 minutes! Changed firmware, reset CMOS clock, etc,. no good. This is what ASUS Support told me -

According to your slow boot, please kindly follow these steps below for how to turn on CSM:
BIOS >> Boot >> Secure Boot >> OS Type
1. To set the OS Type to [Other OS] instead of Windows UEFI mode.
2. Launch CSM to [Enabled]

This really made things worse for me, but after I did that, then managed to get back in again (took hours), and change it back to [Windows UEFI], both dual monitor mode worked and the boot time was back to 15 seconds.

Sorry to hear that. What does your boot sequence looks like in bios? If there are any other drives beside 850 EVO duo, remove them. (I'm almost sure, that you check this too)

Don't be nitpicking! You did it!