Do I have that right? If so, my advice would be to identify the wavelength of that fibre, which can be found printed on the outer jacket. Do not rely on just the color of the jacket, and do not rely on whatever connector is terminating the fibre. The printed label is the final authority.
You got it right!
On the cable unfortunately there is no wavelength printed (it's a cable made for my ISP), but I've read on a forum (that talks about this ISP):
GPON adopts WDM to transmit data of different upstream/downstream wavelengths over the same ODN. Wavelengths range from 1290 - 1330 nm in the upstream direction and from 1480 - 1500 nm in the downstream direction.
Edit: In the meanwhile, do you have any 2,5Gbe PCI card that you suggest (I need it to connect OPNsense to the ONT via PPPoE)? I've found only the QNAP QXG-2G1T-I225 that costs about 75€ or the Edimax EN-9225TX-E for about 41€ (but I haven't read much about this one).