Chances are very good that you have almost zero clay. If you live within a mile of a brick factory, then maybe. The clay like symptoms most people have are due to an imbalance of salts like sodium, potassium, calcium, and magnesium. Once you get those balanced, then the soil becomes almost normal. The best soil test in the country is from Logan Labs in Ohio. Get the $25 test and post the results to the Lawn forum for expert analysis.

Not sure you need a soil test at all and why you would need to report back to the Lawn Care forum when you have opted to remove your lawn is also a bit mysterious :-)) This isn't really a landscape design or lawn care question - if any forum is most appropriate it would be the Soils forum......but you are here now :-))

Here's the deal - unless living close to a major river or on longtime cultivated farmlands, most of the PNW soils are comprised of glacial till. It most certainly contains clay. It can also contain a lot of sand and rocks and is often encountered in various layers of different textural stratas. Although devoid of much organic matter, it tends to be a soil that is well draining and moderately nutrient rich.

I doubt tilling is necessary nor would I necessarily recommend any heavy amending if you are looking to plant mostly natives. If you DO choose to amend then you want amend well before any planting - never individual planting holes and never backfill with anything other than the soil that was removed from the planting hole. That is oldtime advice that has been disproven to be of benefit to plant establishment and can actually deter proper plant establishment. Planting in the indigenous soil is really the way you need to proceed

The bark mulch - if in a thick enough layer - should take you through winter OK. Or you can opt for a bit richer yet still barky soil mix available at local soil yards. If your focus is going to be on natives, you really don't need much altering of the current conditions - they don't need it and they don't really like it either. So shy away from recommendations for a lot of different amendments and ferts. Not necessary!!

btw, most of the greater Portland area is a zone 8 - it is only when you get up into the Cascade foothills that the zones start lowering and zone 6 is limited to the eastern side of the Cascades. If your design is in place, you do not need to wait until spring to plant - you can plant any time now through winter if you like, just avoiding those periods in late winter when rainfall is virtually nonstop. Working water-saturated soils can just create more compaction issues.

If things are, and have been, growing in the near vicinity I would think you will not have much problem with soil and growing things. I would GUESS that tilling would be a mistake and not helpful to you. (But it would be a big PITA!)

What does seem to be a design question, if you care to bring it up, is the pathway scheme. Hopefully, you are/will create a PLAN.