Where are you going to store your boat? At your house in Scapoose? From where I live (Laurelhurst) I primarily use the following launches. As far as low tide, the last two listed work well. You an get a year pass as well for cheap so you can leave your tow vehicle parked for several days in a row. (except for at cathedral park)
Cathedral Park - when I plan on cruising along the Willamette or up to St Helens. The dock is not the greatest but easy in and out most of the time. At peak fishing season it can be a bit hectic
M. James Gleason Memorial Boat Ramp - near marine drive & 33rd, this can get busy but most of the time is ok. Its a nice and well maintained ramp. Sometimes wave runners can be a pain but that season is usually short lived. Most of the time fisherman are out there.
Chinook Landing Marine Park - Out in Troutdale, my favorite launch too bad its so far away... But its fantastic if you plan to head out east. Its well maintained barely need fenders, its protected, no current or waves to worry about. I will launch from here on my first trip.
Anyways, I have launched other places above the falls too. But for proximity to my home these work well.