Since you're familiar with California locations, I'll answer with that. I wouldn't try to ride Mt. Diablo with either an 8 or 9 speed. However, it totally depends on your physical condition and what cluster you have on the bike. You can get cogs for a folder typically running anywhere from 28-32/34 for the largest and typically 11 for the smallest.
If you use an online gear calculator, you can figure out (based on your physical ability) what speeds your gear range will give you.
I ride an 8 speed in the Bay Area - it's fine except for the bigger grades. Anything over around 11% is too much for me, but ymmv, if you aren't nearly 70 years old, lol. If you can wrench a bit you can change out the 8 speed for a different gearing range (and not change the shifters) or change out the shifters and go 9 speed, or change them both along with your chain and go 1x10. The only way you're gonna know for sure is to ride it and see what you need and then make the changes.