My Reno uncle had a golf cabin in Graeagle so I've passed through there a couple of time.
CA89 the whole way to I5 is, I think the direct route without difficulties. It's corouted with 70 and 44 in places. And there are some short cut options around Lassen.
North of Quincy it follows a river valley, so there are curves but no real climbs.
One time I camped at L Almamor (in late May snow) and then down to Red bluff. But the second I took 147 along the east shore of the lake, and then Mooney Rd at Westwood north to CA44. That's a local road, but relatively straight and flat, cutting across the 36/44 angle.
I suppose in full disclosure I should say I left CA89 before I5 to cross the Lava Beds, and on to Klamath Falls (US97).
US395 is another option for going north to Oregon. I'm guessing most of the Reno vacation traffic takes 395 and 70.
In Oregon, US97 is a nice alternative to I5. It stays higher and dryer, with fewer cities.
CA89 south has a good grade south of Sierraville doesn't it? Truckee is quite a bit higher, with Donner pass an additional climb. But you probably are familiar with those. As long as you bypass Lassen, there aren't any comparable climbs heading north.
Or you could skip that climb and go south to Sierra City and take CA49 to central valley. :) Actually I had a pleasant time in Sierra City before opening season, and camping nearby (earlier in that May trip), but haven't driven 49 west.
Ca99 doesn't exist north of Red Bluff, does it? The through highway is I5. Old US99 is either a state or local route, often 2lane or city streets.