But you shall shine more bright in these contents (3)
The "more" here implies that the beloved is already a brightly shining star—at least in the speaker's opinion. The sonnet is only going to add to his brilliance.
The living record of your memory (8)
Although this poem is also about the awesome powers of poetry, especially in comparison to sad, stupid art like statues, the speaker always describes himself first as a lover, then as a poet. In the same way, this poem is primarily a love poem, not a just a poem. Any clever phrasing or memorable imagery exists only to preserve the beloved's memory.
'Gainst death and all-oblivious enmity Shall you pace forth […] (9-10)
This poem is powerful enough to protect the beloved against death. Even if the speaker never says it explicitly, it's pretty clear that the source of this power is the love he feels for the beloved. How else could he write a poem so muscled with praise that it can withstand war and death itself?