Can you download all of the books at once instead of one at a time?
You can. It can be very time consuming though if you have a large library as it can put a strain on the processor installing them (I have an iPad Air 3). So if you're on a limited internet or have a slower connection, you might want to make smaller batches
https://support.logos.com/hc/en-us/articles/360024998132-Logos-Mobile-Download-Resources
You're no longer limited to 100 resources, though you will get a warning if you download > 100 at a time.
I don't see the select feature on my iPad, It seems like I can only download one at a time?
Maybe this will help
tap on select. It will turn into select all
go to the bottom right and click the dots. Choose download
Probably best if you don't.