Hi Renee.
You certainly have time to hit all the destinations you have mentioned in your 6/7 day time frame from Yosemite to Portland. You are right that you could see varied weather conditions, but Crater and Lassen are open year round. There maybe limited access with some roads closed, but it can also be quite spectacular without the crowds. Being a short season, these places can get pretty packed out in the height of season and Crater Lake is on my 'to do ' list in June, with images of deep blue waters and sky's broken by the white of snow covered mountain peaks, in my mind. [Not to say it will be !]
You could leave SF via the Golden Gate Bridge to Muir woods for some Redwoods, or you would need to zig-zag a little to and from the coast and add quite a few miles, if you wanted to visit the Humboltd forest and Redwood NP for example. Altogether a different species, you also have the option to visit the Giant Sequoia trees near the south kiosk of Yosemite in Mariposa Grove.
If you were to go from Mt Shasta to Crater Lake you could consider continuing through Bend OR to Hood River and head into Portland along the Columbia River Gorge.
Hopefully the Tioga pass [CA120] will be open [very good chance] and I would certainly take that route over the Sierras and then up to Tahoe.