I haven’t used the internal mass component much myself. At a glance I think I would change your set up to this
I would also probably make the wheat cylinder float fully contained in the zone.
But I don’t think that will fully solve your issue.
I would probably do a bit of research into how people typically deal with this kind of high thermal mass simulation using energy plus. I had a quick look to see if there’s anything easily accessible out there for simulating libraries for example (with the thermal mass of books) but couldn’t see anything worth sharing at a glance.
It also depends what information you’re trying to get out of the model.
An alternative approach could be to model the wheat as a separate zone. As a closed box zone with no gains, ventilation, or infiltration, it should give the kind of thermal mass effect you’re trying to achieve, although you’d have to tweak with the constructions of that box to get the kind of performance you’re looking for.
I’d definitely be interested to see where you go with it.