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It would be hard for you to maintain two separate branches for a Fabric and one for Forge. Since you need to update to newer Minecraft versions this is a good reason to switch to using Architectury.

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Migrate to Architectury. Architectury allows you to have one loader independent "common folder" for code that is the same across versions and have multiple "folders" for each loader. Each loader specific "folder" will have code that will be used for starting up the mod. This means about 90% of the mod will be inside the "common folder" which will make it easier to maintain two separate versions because they are the same.

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Some people have problems with the Architectury API, however Architectury can be used without the Architectury API, for example you could use Registrate and Porting Lib instead (these are what Create uses and my own mod). https://github.com/Fabricators-of-Create/create-multiloader-addon-template/tree/1.20.1 is a template which shows you how to do this.

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fnuecke/sedna#19 looks like good progress.

I would also enjoy Architectury instead of two separate code branches

I'm hugely in favor of this, considering that it reduces maintenance cost a lot by having common codes in a single branch.

@fnuecke maybe just push it somewhere here maybe we can help port it together?

I also tried to update current branch to more modern version and found out some core parts got replaced with vanilla like patterns

@fnuecke That's excellent news. Do we have an estimation when soon is?

Yes, this is indeed my plan; I had started this way back when I was last active on this project, but lost traction due to some things using specifics of Forge if I remember correctly. Since then I've migrated my other smaller mods to Architectury and feel pretty at home with it. I plan on picking the porting effort back up soon.

(Semi-related, since linked above, I have already pushed an update to the library mod with the virtual page access fix.)

There is https://github.com/North-Western-Development/oc2r which is a continuation of oc2 but ported to modern Minecraft. On their Curseforge page they say they are planning to use Architectury however no code have been made so far.

However, since they have already ported to modern Minecraft it would be a lot easier to port to Architectury by using their code.