I built a SaaS starter kit (https://usegravity.app) and use the same kit in a few SaaS products. I'm obviously biased, but I would never start a new project from scratch for several reasons:

1. Building boilerplate code takes a long time to do things properly and is often never as simple as we may anticipate.

2. The time is better spent on building features for customers.

3. As you mentioned, it's painful to build this stuff.