I grew up in Palo Alto, so I am very familiar with the region. Now, since I live in Seattle, I must admit I am jealous of your over-heating problem!
All kidding aside, I believe there should be a straightforward, and relatively inexpensive, pathway to improving the comfort and efficiency of your home without needing to install A/C.
In the high performance homes (aka building science) world, we tend to address upgrades with a specific loading order. Typically, when balancing costs and benefits, it makes sense to improve:
- the attic first,
- the crawl/basement second, and
- the walls/windows/doors third.
If your house is built on a slab (very good chance in San Jose), then there most likely isn't anything you can do to improve the slab's performance.
So, what should you do for your attic?
The first step is to check if there are any hazardous conditions such as knob-and-tube wiring, rodents, mold, etc. After the attic is cleared for safety, many homeowners still invest in a complete attic cleaning so that all debris is removed and their contractor can start with a blank slate.
Then, you should have a Building Performance Institute certified contractor perform a blower door test to see how much air leakage is occurring in your home. This is a very important piece of data that will inform many decisions such as degree of air sealing and ventilation needed.
Next, the BPI contractor should perform a comprehensive air sealing job in the attic. This will remove the primary driver of most of the comfort, health, and efficiency problems in your home.
Lastly, the same contractor should install attic insulation to R-38 minimum (blown cellulose or blown fiberglass in the most common product choice and please note that roll-out "batts" do not perform well in attics most of the time).
Please note that whole-house ventilation as well as attic ventilation should be discussed with your contractor because you may need additional ventilation based on the air sealing and insulation work.
An attic upgrade as described above is almost always the first step for improving the performance (aka comfort, health, efficiency, durability, and value) of your home.
If budget allows
If there is additional budget, then it would be worth exploring wall insulation, starting with insulating south and west facing walls.
Lastly, if there is still budget available, it may be worth exploring upgrading your entire space conditioning system to something hyper-efficient like a ducted or ductless heat pump system (which would also provide air conditioning).
Lastly, I believe you are in the PG&E Energy Upgrade CA rebate program territory, which would mean you could qualify for thousands of dollars in rebates and quality assured contractors to help you get this work done!
Best of luck and please let me know if you have any more questions.