Numerous times I tried setting the level-type on the server to "realistic", I even tried setting the world-seed and generator-settings to "realistic". Nothing works, it just generates regular old vanilla "default" world. Meanwhile in single player with identical modpack the generator works perfectly fine.
I add my server.properties file here:
https://drive.google.com/file/d/1-ZZxvOfOZFwd0Iba80_Tw2enuB9W2Ui_/view?usp=sharing

Going to need a full game log.

I think you can trivially do it with the new Forge feature: MinecraftForge/MinecraftForge#7448

Make RealisticGenScreen extend ForgeWorldType and register it in its registry event, then you should be able to put its registry name in the Forge config option for the default world type, or in server.properties.

Here's how I migrated Garden of Glass to use that.
Just don't forget to set the minimum version of Forge in mods.toml to 35.1.6 :P