I see some SFP transceivers are still not supported by Linux (e.g. https://gitlab.nic.cz/turris/os/build/-/commit/c90a7049105658502eae7dd0aa89f0f1b20292c9). Could I have a recommendation? The only expectation is that it will work with OpenWrt 22.03 (extra points if it's also known to work with 21.02, but not that important).
Would that even exist? SFP standards support bit rates of 1 or 10 Gbps, a 10 Gb port is called SFP+. There is no provision for 2.5 Gb.
on my switch, using a 10gb sfp and connecting a 2.5gb adapter works at 2.5gb, despite 10gb link on switch side
"In recent years, SFP transceivers have been created that will allow 2.5 Gbit/s and 5 Gbit/s Ethernet speeds with SFPs with 2.5GBASE-T and 5GBASE-T."
But, to be fair, https://en.wikipedia.org/wiki/2.5GBASE-T_and_5GBASE-T
"Whilst Broadcom had announced a series of 2.5 Gbit/s transceiver ICs, 2.5 Gbit/s switch hardware was not widely commercially available at that point. Many early 10GBASE-T switches, particularly those with SFP+ interfaces, do not support the intermediate speeds."