Currently looking into exactly the same topic…
The “Digitalisierungsbox Glasfasermodem” seems fairly popular and is seeing some usage overall, but there seems to be some issue when cold-booting with the SFP installed, as mentioned here: Ma5671A SFP issues on turris os 5.0.3 - #52 by backon
But, according to Hanly HL-GSFP cold boot issue (#413) · Issues · Turris / Turris OS / Turris Build · GitLab , you might be able to work around this issue by setting a a boot delay. Since I do not have yet purchased the SFP (or for that matter: have the fiber line installed), someone else will have to validate this…