Hi there,
So The Part in the post “AMOLED” is a 3 or 4 wire compatible SPI interface display, Things to note from it’s data sheet (sales page) is the Supply Voltage required (Xiao is a 3.3v MCU) Display Driver chip:SH8601 or compatible and the Touch screen IC Driver required for FT3168 or compatible., Seams pretty standard albeit you will need to read up on what pins connect to which.
Yea , none of your proposed code implementations raise any concerns as I see it.
just make sure you have a library that works with what you are using. 
The battery size could be anything from 400mAh, to 1000mAh based on size and total run time required. We/I often recommend the Battery Life Calculator from Digi_key’s page it works well , Plug in your numbers.
Makes it too easy.
HTH
GL 
PJ 