I was going to put in 2.5" SATA SSD in my case, but then I saw the power cables and data cables and I eventually decided to go with 2 M.2's.
Original plan was 512Gb M.2 + 256Gb 2.5" SATA
Final result was 1Tb M.2 + 512Gb M.2 + no extra cables/management needed!!
I would recommend you to go with at least 1 M.2 to use for your boot disk. Startup is insanely immediate.
Do you have ASUS B450 board? What are the temperatures like of M.2 drives?
Edit: Noticed that you have ASRock Z390 in another post