Best theme/boiler plate for WordPress theme development in 2021?

I think I’m having an existential crisis. Six years ago, I began building WordPress websites using a pre-built theme, and I would just modify the child CSS, page builder, etc to make it look like my clients’ brand. Eventually, the sites started to get larger, with bigger budgets, so I had time to explore better options. I jumped on Underscores (_S), which is good, but there were lots of components I had to rebuild over and over. I switch to Understrap which I still like, but development has been slow, and I’m scared I’m going to become obsolete.

My question is, what is your build process for 2021 for larger projects (ones that take a few months overall) and what’s your Workflow?