.. an excellent working practice 
After some consideration, I'm finally dropping my pursuit of replacing ISP ONU with GPON ONU stick as there are simply not enough available SFP+ ports on the router.
That, plus the fact that my current ISP uses both GPON and XG-PON standards make purchasing GPON/EPON ONU stick a losing investment, therefore it is highly likely that I will continue to use the ISP's equipment for quite some time until at least a major upgrade.
yeah, i decided against it too