These are the road routes going from Portland to Austin. The simplest would be to get on the Pacific Coast at Portland and ride it with the tailwind down to San Diego. Then get on the Southern Tier and ride it with likely winds out of the west to Austin. Another option would be to take the Pacific Coast down to San Francisco, then the Western Express to the Sierra Cascades route. Go south on the Sierra until it ends at the Southern Tier near San Diego. Then the Southern Tier to Austin. This would give you some mountain riding and maybe avoid some of the heat along the coast in southern California. But the problem with both of these routes is you are riding the routes at the hottest time of the year. June-July. Not sure how you like riding and camping in heat. One other option is the Pacific Coast down to San Francisco. Western Express to its end in Colorado, south-middle. Then use state maps to pick out some county roads to get you down to Austin. From Colorado to Austin its about 600+ miles. Solid week of riding.
I did not see you wrote the following:
"should I head east after san Fran to avoid the southern/ I-10 heat that time of year? Ideally I'd like to go through Utah/salt lake and Colorado."
If this is a goal then by all means take the Pacific Coast down to San Francisco, take the Western Express to its end in south/central Colorado. Then county/state roads down to Austin. Not sure going through Nevada and Utah will be any cooler than southern California. Expect the heat will be identical.