Can I assume there will be no problem accessing my Drobo USB2 drive via the USB3 port on the DS218+? I currently access this via the USB port on my AirPort Extreme, but since there is no functioning USB port on the Eero Pro routers my plan is to attach the Drobo to the DS218+
Yes, based on many people experiences, you can connect Drobo and other external RAID DAS boxes (hardware RAID only) to Synology via USB. Just make sure it comes with supported file system.
By connecting an external disk to the system, you will be able to share its disk capacity through a system-created shared folder named usbshare[number] (for USB disks, if applicable) or satashare[number] (for eSATA disks, if applicable). The shared folder will be removed automatically when the external disk is ejected from the system.
Synology NAS recognizes the following formats: Btrfs, ext3, ext4, FAT, FAT32, exFAT, HFS, HFS Plus, and NTFS. Any unrecognized external disk will have to be formatted first before being used on the system.