I love driving up 395. There's a lot to enjoy no matter what time of year you do that trip. I particularly like spring and fall though, but you do have to watch for late/early snow storms since there are several high passes.
For National Park sites, there's Death Valley NP of course, Manzanar NHS (the Japanese internment camp you mentioned), and Devils Postpile NM. All of them are well worth a visit depending on the time of year. Death Valley is horrific in the summer and Devils Postpile is only reachable in the summer.
There's a small cabin resort called Red's Meadow in the valley just on the other side of the mountain from Mammoth Lakes that's a fun place to stay for a night or two. It's mostly frequented by hikers doing the Pacific Crest or John Muir trails so you meet some interesting characters, but you can easily get over to Devils Postpile from there or hike to Rainbow Falls.
Definitely stop at the Ancient Bristlecone Pine Forest.
North of June Lake, we did a great hike in the Ansel Adams Wilderness up to Parker Lake.
If you continue along 395 all the way up past Reno and back into California, make a detour to the rail museum in Portola. You can wander among the train cars, some of which have been restored, or you can make a reservation to drive one of the engines.
Once you get much north of there you're in the southern Cascade Range rather than the Sierra Nevada Range, but it's still a great place to explore. In addition to Lassen National Park, there's Lava Beds National Monument which is somewhat less busy. I've heard Burney Falls has become an influencer hangout of late, but it's still worth a look if it's not too crowded. Hat Creek is a quieter area if you want camping and fishing.