I’m using NextCloudPi on a Raspberry Pi 4b since January '20, and regarding the software, I’m pretty damn happy!
Currently, it serves files from 3x4 tb hard drives (as one btrfs) on a USB 3.0 hub with it’s own power supply.
However, I’m now looking for faster hardware (and would repurpose the raspberry for something new).
I am wondering if a) the ODROID XU4 is suitable and b) worth the price ~95€.
The sbc boards are similar (since you already have a nice setup). Biggest speed gains will be moving away from usb to actual sata connections over PCI-e. And migrating to an SSD or NVME drive. And be sure to use wired gigabit interfaces locally.
Also, upgrade your internet service if you can to get faster speed. Perhaps your area has fiber internet available as true 1000/mb from a local isp.
In the end, only an actual x86 system will give you truly insane speeds. Arm support always lags behind in Nextcloud. And I continue to use a cheap device so I get it. Don’t know anything about your setup, but it is just up to you. You will be shocked at how much faster a full blown box is.
I’d focus more on backing up your system and improving your actual network speeds then worrying about replacing the Pi 4 with anything other than adding a faster SSD for improving data sync access.