Jeff -
If you are leaving from Astoria on June 1, there is only one pass that you have to be concerned with - McKenzie Pass in Oregon which is east of Eugene. At the pace you describe, you should not be hitting McKenzie until June 10-12. In previous years the pass usually opens by mid June unless there is major construction or a massive snowpack. In addition, cyclists are often permitted to use the road a couple of weeks early. So there is a good chance you will be able to cross McKenzie Pass. If McKenzie is still closed, the detour is via Santiam Pass 0 a little further, a little lower, a good deal more traffic, but with shoulders. No problema.
As for Seattle to Astoria -
The ACA Pacific Coast map section between Centralia and Longview is rather meh. And Hwy 4 between Longview and Cathlamet is somewhat busy and narrow - but scenic. (I did graduate research in Cathlamet.) A more direct route is to cut west from Elwa to South Aberdeen on back roads, then US 101 South to the Astoria Bridge. The Astoria Bridge is 4 miles long (6.5 km) without shoulders - some cyclists take the bus across, instead. Two good extras on the US 101 option - a) Take Hwy 105 from South Aberdeen to Westport and then follow around Willapa Bay, the Willapa Bay ride is stunning. b) Instead of taking the Astoria Bridge, ride east on Hwy 4 thru Skamokawa to Cathlamet and take the ferry across the Columbia River. Skamokawa has a county park with the finest views of the Lower Columbia, you can ride along the river's edge thru the Columbia White-Tailed Deer Refuge, and Cathlamet is an old river/fishing town.
And bears -
I never worry and have ridden solo up to the Yukon and Alaska, lived in Wyoming for 25 years. Two key points, though - 1) Never, ever eat in your tent - even if you are not going to be in bear country for weeks. 2) Always store your food and aromatic items in bear boxes or hanging from poles. Carry 50 ft. of nylon cord and learn how to hang when boxes/poles not available. Even if you don't take these precautions, your risks are minuscule. However, if a bear does learn to steal human food, it will be destroyed - - not you.
Have a great trip.
Jama