I’m no expert but a few things stood out to me.
You could try to include some texture work - bump maps + image textures are a simple way of adding extra detail.
I’d also consider deciding which parts are painted, and which are bare/anodised metal. For tthe painted surfaces you could use a more realistic “paint” pbr with texturing. Depends if they are meant to be factory new or battle-worn. I always thought the armour was painted, so your finish here would be too metallic.
You can see what can be added here (Anvil) for example - the base mesh is relatively simple, but a lot of detail is added “for free” with a suitable bump map and some simple texture painting.
What is the thing in the background?
Lastly, not a problem with your work, but I always thought GW needed to make the magazines on bolters bigger. They always look like tthey’d hold barely a single bullet! (same with grip - also seems small for an 8ft tall armour-encased demigod).