hello,
sorry for such late response, I was on vacations.
Well, it is very hard work in front of you, as GPON SoC's are not just a simple ethernet pass-through devices.
The most difficult part to handle will be a GPON PHY/SERDES. As you can find some SDKs in internet (leaked) related to in example realtek's router chipsets, but - these chipsets do not support GPON.
Without documentation and often - vendor's support it is very difficult to fix some specific issues, as sometimes you need to set specific bits in specific registers.
I was trying once to make software for SFP module with RTL9601C onboard, and I gave up for a moment, as I have no circuit diagram that is necessary to correctly specify GPIOs for laser signals, reset, link-up/dow and several others.
Once I have some more time, I will also focus on such product, but - only if factory provides full GPIO specification or circuit diagram for this specific unit.
And with SFPs if there is no serial connector, you need to remove and programm flash each time, that is time consuming and it is easy to destroy all device as it is very little.
Generally I do all that staff with Realtek SoCs with normal ONTs/ONUs, so based on 9601D (also can do 9601B, but this is old chipset not too good), 9603C, 9607C and some newer I have not seen, but are supported by recent SDK.
Latest chipsets use ARM core instead of MIPS.
Comming back to your OLT for lab - if this is supposed to be cheap - think about used Dasan V5812G or V5824G.
I have seen V5824G sold in Poland for about 3k PLN (+ VAT or no VAT if this is inter-eu b2b transaction).
For lab - it is ok.