Deck posts near tree with roots

I am thinking how to put deck posts on a very slightly sloping ground -- a 10X12 ft deck and about 2 ft high. For two of the four posts I can dig holes, pour concrete and put the 6X6s or post holders without a problem. These posts are at the upper end of the slope. The other two posts are nearer to a huge 100ft cedar and so there is very little space to dig 3fts without hitting rocks/roots.

I am thinking these two posts can be floating ones -- with 4-way concrete blocks for the joists. However, even that maybe hard because of the roots -- not enough flat space to put the blocks solidly on top of the roots or near there (it seems).

The last option is to dig a straight 2 ft hole and insert a adjustable deck support (right angle for the rim joists at the corner). But it is barely 1.5 ft max if I extend it using its screw mechanism.

Any suggestions would be very very appreciated.