Which type of shadow mod do you use?

Iris + any shader pack of your choice will serve you fine. As a side note, ever since OptiFine got obsoleted by Sodium et al, Iris has become the new go-to shader mod. Sure, you're downloading a suite of mods instead of just one mod, but in terms of optimizations they beat out OptiFine by a very, very wide margin.

Though I personally recommend playing the game without a shader pack.

Spoiler: Tested Shader Packs

These are the shaders I tested the game with - the list is not exhaustive. GPU is Arc A750.
Complementary: By far the most straightforward of the shaders. Works out of the box, even if you have a potato GPU, and gives good performance. Incidentally, it's one of the few shaderpacks that don't turn into a glitchy mess with nametags.
Photon: I personally couldn't find a difference between this and Complementary, with Photon having less FPS over Complementary, though a more experienced eye might be able to tell the difference. Works fine with nametags.
Super Duper Vanilla: YUK. The visual fidelity is fine, but it glitches out with nametags.
UShader: The game is extremely blurry and washed out, not recommended at all. Also has the nametag visual glitch.
FastPBR: Nametag glitch. Nuff said.
Exposa: Not recommended at all. Clouds look wonky, there's a persistent fuzziness on the screen (except the GUI), and the nametag glitch is once again present.
Arc: I would be recommending it if it wasn't for the fact that the shader runs afoul of the nametag glitch as well. Some darker areas in cities like Detlas also turn pitch black.
VECTOR: Meme shader used exclusively for the monochrome effect. Not recommended for the faint of heart.

Also let's conveniently ignore the fact that so far only CT and Staff members have responded to this thread