As fttmeh says, going for the SFP solution you might face a steep learning curve, mostly for nothing if you are not interested in breaking the 1Gbps uplink barrier.
You won't anyway if it is to plug the SFP ONT to a device which is not HGSMII (2.5G) compatible. There's really not so many.
Besides, i would strongly advise against Alibaba/express , as you will likely get ZTE-clones which -might- sync with orange OLT, but are so badly manufactured, some even jamming the laser signal, bringing down the whole neighborhood periodically.
If you are just seeking to replace LB5 with another router, just unlock the Huawei ONT, replicate LB5 serial, and go for the router solution that suits you most (Ubiquiti, Mikrotik, openWRT, xxSense, ...)
Keep in mind that even the router setup alone is far from trivial because of some orange specifics (VLAN 802.1q priority for DHCP requests only, exotic DHCP options, auth string calculation).
The overall is very instructive but it's up to you to decide if it is worth the journey
