TileEntity

Forge requires Forge mods to work, NeoForge is not the same

Embeddium is by default not compatible with Veil

Hm I have no idea

Yes If Create gets an update, most of the addons have to update, too

They do work together - but not all builds are compatible

The latest Create build may not work with Create Aeronautics

Use this as working base and just add some mods

Does this modpack work? https://www.curseforge.com/minecraft/modpacks/distant-horizons-iris-shaders

Delete the config files of these 2 mods (config folder) - if there is no change, use a pre-configured mod pack with these 2 as working base

Does it work with only these 2 mods, without an other mods? If yes, add the rest of your mods one by one or in groups

Check for updates of these 2 mods - maybe there are newer 1.21.1 builds of these

Try other builds of these 2 mods

Maybe some sodium addons (sodium options api etc)

Does it work without Iris?

Remove iceberg, Toggle Enchantments, Legendary Tooltips and Quark