Help me understand the structure of this sentence from Virginia Woolf To the Lighthouse

I have some knowledge of English syntax with tree diagrams. Now, I’m learning traditional sentence diagramming. Of course, I want to challenge myself, and I chose to understand a sentence from Virginia Woolf’s novel To the Lighthouse. The sentence is:

Since he belonged, even at the age of six, to that great clan which cannot keep this feeling separate from that, but must let future prospects, with their joys and sorrows, cloud what is actually at hand, since to such people even in earliest childhood any turn in the wheel of sensation has the power to crystallise and transfix the moment upon which its gloom or radiance rests, James Ramsay, sitting on the floor cutting out pictures from the illustrated catalogue of the Army and Navy stores, endowed the picture of a refrigerator, as his mother spoke, with heavenly bliss.

I figured out we can see it as a subordinate sentence and the main sentence:

[Since he belonged, …], [James Ramsay, …, endowed the picture of a refrigerator, as his mother spoke, with heavenly bliss.]

The main clause I understand. The subordinating clause on the other hand is quite confusing. I can remove the main clause and make the subordinating clause the main clause for a simpler analysis. I can also remove some nonessential phrases:

He belonged to that great clan which cannot keep this feeling separate from that, but must let future prospects cloud what is actually at hand, since to such people any turn in the wheel of sensation has the power to crystallise and transfix the moment upon which its gloom or radiance rests.

Now I see there is a compound "which" clause which I can break into two independent clauses:

He belonged to that great clan which cannot keep this feeling separate
from that. He belonged to that great clan which must let future
prospects cloud what is actually at hand.

Now, I am confused about the remaining subordinating clause:

since to such people any turn in the wheel of sensation has the power to crystallise and transfix the moment upon which its gloom or radiance rests.

Does the above subordinating clause go with:

He belonged to that great clan which must let future
prospects cloud what is actually at hand, since to such people any turn in the wheel of sensation has the power to crystallise and transfix the moment upon which its gloom or radiance rests.

Is this correct? What type of clause is this one?