This is absolutely something you'd want to do, and this post answered my challenge!
My problem is with all the various books, lesson manuals etc. When you get a new device and install Gospel Library nothing is installed. No Scriptures, no primary manuals, no conference talks, no Ensign magazine editions. They're all grayed out. When you click on them, they then download. The problem is that if you want all sessions of conference, you have to go one by one, each 6 months all the way back to 1971 and click each one. That's 46 years x 2 sessions per year = 92 times.
Magazines go back to 1971, and there are 12 of them each year. If you want all magazines cached you have to click on each month's edition to download it ONE BY ONE. That's 12 times per year from 1971 through 2017. That's 46 years x 12 months means I have to manually click 552 Times!! Note that I also get a new phone every year, so each year I get to go and click through hundreds of things to manually get all the material back on my phone.
The problem is if you don't have these downloaded locally, when you search for a topic, say GRATTITUDE, you will only see content returned where you have the material download. So if President Kimball gave an amazing talk about Missionary Work, you won't find that unless you have it cached / downloaded.
I had no idea that you could long press on an area and download all the magazines. That's great news. Now we need to raise that up a level and enable it to download all content for the entire app. THEN, I want an option to download new content as it becomes available so that i'm not having to go download new magazines, and conference as it updates each time. I also have a 256GB phone and have plenty of room. Then again, most of this is text, and does not take up a ton of space. If Videos are included there should be a separate option whether to cache videos as this would take up a lot more room.