Newbie First Build (NK4.0) with a future pfsense (mini-ITX)

I am a Noob. I continue to harvest knowledge on this site! I am about to pull the trigger on the NAS Killer 4.0 Ultra Quiet Build version (NK4_UQB), since the server needs to be in my living area.

A. I have a Fractal Design Define 7 XL Case. Is there any benefit of taking advantage of a full ATX vs the MicroATX board since I have the room in the case?

B. I plan on creating a mini-ITX pfsense build and enclosing it in the 7XL Case as well. my below build has an estimated wattage of 244W but am purchasing a SeaSonic 550W Focus 80+ Gold.
Will this PSU be able to supply power to both pfsense (mini-ITX) and the MicroATX board or is that stupid? Is best practice 2 separate power supplies?

C. I know my case has capability of USB 3 ports. Is it easy to connect my USB 2 on the mobo with the USB 3 ports on the case? (basically to take advantage of the usb ports)

Sorry, I don’t know how to make a “prettier” chart of my planned build.

My Build so far:
Case: Fractal Design Define 7 XL (ATX)
Motherboard: X9SCM-F-O
CPU: Xeon ES-1270v2
CPU Cooler: Arctic Freezer 12
Power Supply: SeaSonic Focus 80+ Gold
SAS HBA: Postponing until have more HDDs
SAS Cable: Postponing until have more HDDs
SATA Cable: 6 Pack SATA cable
Fans: Arctic P12 120mm 5 Pack
Storage: Seagate IronWolf 3.5" 1x10TB HDD 7200 (Parity Drive)
Storage: Seagate IronWolf 3.5" 2x6TB HDD 7200 (Data)
SSD: Team GX2 512GB SATA3 2.5"
SSD: Kingston A400 120GB 2.5"
Fans: Arctic P12 5-Pack

Am I forgetting anything?
Does the future pfsense plan make sense inside the Fractal Design Define 7 XL Case?

Thanks for any insight!