@eyegeegeewhy I bought a Gen8 G2020T because last December there was pretty cheap offer I couldn't pass by (~175 Euro). I did a lot of read ahead before the purchase and still missed some info. I'll try to help you, just know that I'm in no way a pro like Spy Alelo, just your about average user.
1.) I see lots of old posts saying in AHCI mode you cannot boot from the OOD port. Is this still the case with the latest bios?
2.) if it is the case, how would you recommend setting up the disks to be used by OMV
You can boot only from the first populated slot in AHCI mode. ODD SATA port is 5th so you can only boot from it when the drive bays are empty. The ways around this I've read about so far are to either run in legacy (which is not good for SSDs) or RAID and use 1 drive Raid0 for boot (that's not good if you want direct access to the block device, for instance if you plan to run RaidZ or md and if you lose the raid you might lose your data). Other way around is to get a PCIx RAID and use that for the drive bays and only use the onboard in AHCI with only the boot drive. Personally I run SSD boot in cage 1 and 3 HDDs in the next 3 and plan to get a LSI 9211-8i or similar for HBA and connect it to the drive cage and leave the SSD on the B120i in AHCI. Also I've read some guys were able to boot one of the versions of Windows Server from USB used just for the booting part and still use a drive in the ODD as system drive. I'm fairly sure it can also be done with grub and a USB or microSD, but I don't have the time to mess with it now.
3.) if you don't use the raid is it still true that the fans run faster/louder than they do if you do use the raid.
From what I've read HP have worked on that and it's better the in the begging. I'm not sure if the fan is louder in AHCI but I my Microserver is in the same room I sleep and I can barely hear it. It is more silent them my tower PC which runs silent fans. The only time I've noticed it's loud is on boot and that's rare if you plan to use it 24/7.