[Citroënbender]

Mulch won’t do it. You need a retaining wall that incorporates drainage. The DIY stackable block systems are unlikely to last in such a loaded application unless expertly prepared, drained and laid. Rock walls start off with huge stones at the base, again not an amateur job to prepare and lay the base course.

Look at some of the systems using universal beam (“I-Beam”) sections set into augured holes, with concrete faux sleepers slotted into the beams’ webbing.

Also, despite being the least appealing way to go, start with your furthest walls and levels. That way any equipment you bring in won’t damage other landscaping works.