It's somewhat inadequate based on my testing with iperf3. The CPU usage runs high even with basic routing between two interfaces and no VLANs, PPPoE encapsulation or NAT. Individual connections tend to slow down for whatever reason when there's a high amount of connections being tracked. You also need to make sure you use ports 1/3/5 and 2/4 for your WAN ingress/egress due to how the components are wired or you might be bottlenecked.
You should be able to swap your GPON ONU with other units from Aliexpress as long as the brand matches and you have the 10 digit authentication key from your existing unit. Unlikely a GPON SFP module would work unless it's from the same vendor and they allow you to edit the authentication key via the SFP I2C bus or an IP interface. I've already done this on the Fiberhome OLT's with a tiny single gigabit port ONU I picked up but YMMV with other vendors.