The looking at the specification sheet of this excavator for example, the maximum digging depth is more like 1.5m. So you can probably do it by digging a 1.5m deep hole but longer than you want with a gradient up rather than a step, then digging another 1.5m deep afterwards where you want.
Digging near tree roots is tricky a digging bar and sharp spade will help more than an SDS drill. For the brick store, fixing posts with proper masonry bolts should be fine if the wall is solid. Don’t forget drainage at the base of the posts.
What a lot of people do in situations like yours is dig and build in sections. You could dig a metre-wide trench where the retaining wall will go, get the footings poured, build that section of wall, backfill behind it, then move along and repeat.