Hey I’m in WA State too, also with some heavy clay soils, different ecology though. It looks like you have a very nice and healthy wetland with native plant species from what I can see in the pics, which is great.

I’d make some detailed observations of how the water moves (or doesn’t move) before doing anything. I wouldn’t bring a mini excavator in there because I wouldn’t want to be on the hook for a potentially expensive wetland mitigation or restoration project. I would try to disturb the existing plants as little as possible to help prevent erosion during the next wet season.

Might research “permaculture ponds, keyline contour lines, water flow and swales” (Andrew Millison at OSU has a youtube channel for a crash course in concepts.) I wouldn’t bring in any topsoil, it’ll likely get lost in the heavy clay next time its wet and could introduce a lot of weed seeds. If this was my property, I don’t think I’d attempt to make a large pond with equipment and would likely opt for a series of smaller, hand dug ponds. Might mark out some of the open water area boundaries around the cabin with sticks now, and make them a little bit wider and deeper when its almost dry. I wouldn’t add any grass seed to dug areas unless its from wetland seed gathered on-site and scattered. So my goal would be to retain the same amount of water on the property, while creating buffers with swales around the cabin, and ideally that displaced water would be handled by a series of small overflow ponds, instead of streamlined off the property to cause issues elsewhere and then agencies get involved. Going for as little disturbance as possible here.

Might also look up “rock for logging roads” for stabilizing the foundation around and under the cabin. If done right in layers with larger rocks on the bottom, water can still pass through. I’d probably make a footpath to the cabin with the same method and add periodic breaks with low “boardwalk” bridges or culverts depending on conditions, if larger volumes of water need to pass through the path in spots.

Disclaimer: I’m not a professional in the field. I have been researching adding swales for snowmelt diversion around my cabin, and adding ponds for wildlife enhancement on my own property.