I was also thinking Miaoli, more around Nanzhuang though as Taian is very prone to landslides.
1000m up is going to make finding a non-landslide prone area difficult. At least in terms of reliable road access. I'm sure there are lots of stable bits of land, but the roads in are often washed out.
The gov is supposed to put out a report in the coming years on all the unstable geological parts of Taiwan. I would wait to see this before getting my heart set on any area.
If you are willing to lower your sights to 400-500m you have all the hills around taipei: Maokong, Shiding, Pingxi and even Pinglin. They can be much more comfortable in summer than the city.
“Everywhere else in the world is also really old” said Prof. Liu, a renowned historian at Beijing University. “We always learn that China has 5000 years of cultural heritage, and that therefore we are very special. It appears that other places also have some of this heritage stuff. And are also old. Like, really old.”http://hikingintaiwan.blogspot.com/