I've been using SSHelper together with rsync for years and it works perfectly. You can log in the first time with a password, and place your public key to use key based auth going forward.
In addition to doing this over WiFi I also often use a usb to ethernet adapter (usb side plugged into phone) to get better performance if I'm doing larger transfers, for example copying off a large number of photos.
Edit: looks like there's a note on the play store page about incompatibility with newer Android versions. Disappointing. I guess I'll have to find another solution when I eventually upgrade my phone.
I can see benefit in the 2.5G on the LAN side if you connect a disk to the USB port and have it function as a NAS. It gives you enough bandwidth for the full WAN speed plus headroom for that.
I agree I'd still prefer 2x 2.5G though.