It’s full duplex so it’s 2.5Gb each way simultaneosly. Most NICs support half-duplex but I don’t know of any good reason to use that. I used to have a BananaPi based router that could comfortably saturate it’s gigiabit interface. I assume there’s some kind of offloading going on.
IMHO, security updates are more important than OS updates, and Fairphone is good in that regard. I’d be hard-pressed to even name a killer feature from the last few versions of Android (or iOS, for that matter).