I suggest you be mindful with how much plantings you rip out before replacing with something--stone, other plantings, etc. You so not want to have water washing your supports out or creating issues.

THis is way beyond this forum you need prop help and this can be done in stages but you need a plan and I agree riping all the stuff out can have huge implications . Stop now get a landscape designer who will work with you for the next 2-3 yrs to do this right . I am not seeing terracing with actual proper retaining walls and if this is to become useable that is what you need

For most of this, you want a cheap, shade-tolerant groundcover. For access, we have to ask access to what? Are you going to have a usable tier?