OMG - August in the Central Valley is brutal.
And your profile says "GB" which I presume is Great Britain,
not the Great Barrier Reef in Australia.
Aussies can handle 45C / 113F - - not so sure about Brits.

Then, too, August is high fire season in California.
The chaparral scrublands of the mid-coast and the Sierra foothills are tinder dry.
This winter is looking to be the third wet winter in a row, so fire danger may be less.
But it will be VERY hot any distance from the coast.

If you are planning 3 months, any chance you could start 2 weeks earlier?
Yes, you are more likely to have wet weather in Washington/Oregon.
But 3 months seems quite a long time for the Pacific Coast Route.
1500-1600 miles. Even at 250 miles per week is 6-7 weeks.

I know it is morally suspect but,
One option is to ride California N-to-S,
then hop up to Vancouver and ride the Wash/Ore stretch.
Riding south to north puts you up against prevailing winds.
(Multiple entries into U.S. can be a visa issue, too.)

The Google map looks O.K. Specifics may vary a bit.
It is a low to very low traffic route.
Camping/lodging is sparse in places.
Sunset State Beach to Mercey H.S. is doable.
But Mercey now charges $75 - it was $25 years back.
Firebaugh and Chowchilla have motels. Mariposa is pricey.
The Yosemite Bug Hostel also used to be cheap.
It's a bit closer to Yosemite than Mariposa.

Is it possible to fly into and out of SFO?
Ride San Fran to San Diego -
Then take Amtrak to Merced - easy to take bike on board -
Ride into Yosemite and back to Merced.
Then take Amtrak up to Vancouver overnight.
Finally ride back to San Fran.

The waterfalls in Yosemite are spectacular in May/early June.
They are only a trickle by August.

Just a few ideas. Ya gotta shake 'em up from time to time.

Jama





Pic - Yosemite Falls at Peak Snowmelt in late May, Valley Still Flooded