OK I finally got this working. This FS Generic Compatible 2.5GBASE-T SFP Copper RJ-45 module does indeed work: https://www.fs.com/products/139650.html
I dont know if it would have done it automatically or not but based on the comment above, I relinked the default device tree symlink and then rebooted and it worked without further changes to setup after that:
cd /boot/
rm dtb
ln -s armada-385-turris-omnia-sfp.dtb dtb
reboot
After reboot:
# dmesg | grep -i -e sfp -e eth2
[ 1.823696] mvneta f1034000.ethernet eth2: Using hardware mac address d8:58:d7:00:34:81
[ 11.812748] sfp sfp: Host maximum power 3.0W
[ 12.137957] sfp sfp: module FS SFP-2.5G-T rev 1.0 sn G2230090032 dc 220711
[ 12.147352] mvneta f1034000.ethernet eth2: switched to inband/2500base-x link mode
[ 20.454622] mvneta f1034000.ethernet eth2: configuring for inband/2500base-x link mode
[ 24.735381] mvneta f1034000.ethernet eth2: Link is Up - 2.5Gbps/Full - flow control rx/tx
[ 24.735412] IPv6: ADDRCONF(NETDEV_CHANGE): eth2: link becomes ready
This was using TurrisOS 6.2.3 .