In your shoes I’d take a look at “Migrating a UEFI-based install” chapter and this advice on “Installing PureOS on a 2015 MacBook Pro” (quite sure this is posted somewhere on this forum).
Another explanation is here:
As you already understand that people usually do not have enough time, have another priorities, etc., please try to share with us that you understood process/difference when installing PureOS and return here with some kind of solution, please, when at least half way done with it (even if this is the only computer that you can use, and prepare necessary, in order to make things happen).