Steamboat Springs, CO

1) Located in the western US, preferably no further north than Northern
California

Yes

2) Base camp no highter than 8,500 feet.

Yes. Base elevation 6,900, top of Mt. Werner 10,568

3) Easy access to hot springs, warm swimming pools.

Yes. Rec center in town, a couple of miles away. Fed by natural hot
springs. Strawbery Hot Springs just outside town in a fairly natural
state.