All of those HP minis have 2 NVMe slots. If you're looking for more bays maybe a QNAP TBS-h574TX (Core i3-1320PE) will bit your needs better. Or the Asustor Flashstor 6 FS6706T.
But I want 1 boot drive and 2 x storage drives in RAID
One other possible approach to this is to go with the 800 G4 or the 800 G6 as they also SATA port you can use for your boot drive.
You can also boot from a fast USB 3 flash drive, since it's your boot drive it won't be as bad as you think. Consider some servers boot from SD Cards and other low performance media with almost static images.