Looking for help with BiDi MMF SFPs, media converters or other alternatives to replacing MMF with SMF

Two of our student dorms are connected to the campus with a radio link. On the campus side we operate the transmitter, power supply and a ~20 m 2-core MMF connecting it to a patch panel in a utility room on the top floor. There our 2 cores are connected to a university-operated MMF running some 20-30 m to the building's core-switch.

The radio link should be able to transmit 2 to 2.5 Gbit/s over this distance, it only has 1 SFP-Port though and the manufacturer told us, that we'd need a BiDi-Duplex-SFP transceiver for SMF to be able to transmit more than 1 Gbit/s. Due to some issues with 3rd party network equipment in the past, we're not allowed to operate a switch on campus, a media converter would be ok though.

Our 20 m MMF has to be replaced soon and an estimate for that came at ~1000 €. To replace both our and the university's MMF with SMF, we'd have to pass an additional ~6 floors/ceilings. The dorms are operated by a non-profit organization and I'm quite sure they won't see the benefit of paying triple the required amount to fix our cable (my estimate, still missing some specs to get an estimate) for bandwidth we currently couldn't even use as our bandwidth is limited to 1 Gbit/s by the university's firewall until it gets upgraded.

To save some money - or have an alternative if the board rejects the extended replacement - we could

1. place a media converter in between to convert from 2x 1 Gbit/s BiDi SMF to 1x 10 Gbit/s MMF or
2. use a 2x 1 Gbit/s BiDi MMF SFP transceiver, if those exist.

Media converters with 2 SFP-Ports that can switch between different bandwidths are hard to find and the ones I found cost beyond 1500 €. Did I just not look in the right places or are these devices that expensive?

On another thread u/Gesha24 linked a BiDi-MMF transceiver, which is only simplex unfortunately. Are there also Duplex variants of these? On the net I found this 2.5 Gbit/s transceiver. Is that what I'm looking for or is it just 2 links with 1.25 Gbit/s each only transmitting and not receiving any data?

I'm a little in over my head here and would really appreciate any recommendations have or alternatives you see :)

I know these aren't the dimensions usually discussed on this subreddit, but I hope it's more in this direction that home networking. All in all, 250 residents would be really grateful - I assume - if they weren't stuck on 1 Gbit/s for at least the next 5 years.