If I start using a specific tense, do I need to continue to use that tense throughout a sentence/paragraph? Can I write the second sentence as below or do I need to make it the same tense as the first sentence?
Sarah had spent almost half her life in England. She grew up there, got married there, and had most of her children there.
or
Sarah had spent almost half her life in England. She had grown up there, had gotten married there, and had had most of her children there.
No, I think your first quote is fine. I find that in most cases, what a past perfect really does is push your whole mindset back in time. So when you say "Sarah had spent half her life..." I start thinking, okay, so we're talking about that now, Sarah as a younger person. "She grew up there, got married there, and had most of her children there." That's all in the time frame I would expect, young Sarah.
I would consider whether what comes next pulls you back to the time of the story well, though. Something like "But in all that time, she'd never had a day like this." or "But today it felt like she was in a completely different country." Something that lets you flow back to the "present" when you're done setting the stage. And then you're okay to use the simple past again to refer to the narrative time.
Sarah had spent almost half her life in England.
Full stop. The overall tense should remain in the past but you are free to put your words in motion. A common transition is to go with the participle.
After growing up, getting married and having children, Sarah decided it was time to move on.
These are the basics of story telling. How can we write engaging fiction if we are stuck with one tense for 200 pages? No thanks!
