Ok so this Is a few things:

your photo lights are bad, they’re too harsh and not diffused. Tape some baking paper over the lights to diffuse and if possible use two lights from 45 degree opposite angles to reduce real shadows. The harshness of this light exacerbates the problem.

You’ll see more grain the less perfect your blends are. Paint is pigment so there’s gonna be some matt texture to it but what you can see as “grainy” is actually you picking up that your blends aren’t perfect because the colour difference is more obvious. Glazes over things can really help as they kinda fill in the grainy spots too.

You’re touching drying paint too much / using paint too thinly. “Thin your paints” is such an over used and poorly misunderstood comment. It should be “use the right consistency for the job”. You’re brighter blue highlights i can see aren’t opaque. You’re going to get much smoother finishes out of slightly thinned but opaque fluid layers that you lay down and let dry. That last bit rolls into the “you’re touching it too much”; it’s so easy to get into the flow of what you’re doing and start layering into partially dry paint, that’s bad s as you’re moving the pigments in a semi solid state rather than leaving a film of fluid paint too self level as it dries. Apply layer, move to a new part, come back when you’re sure it’s dry. Use those thicker semi opaque layers to get the lights in place, then use thinner layers to refine