Can I arbitrarily select a template from https://startbootstrap.com/ and use pn.template() on it?
Yes. I have for a long time wanted to do the same. To showcase that it is powerful and how powerful it actually is.
But take your time to find the best template out there. There are many cool ones. Also non Bootstrap ones. For example there is a longer list here 40 Best Free & Responsive Admin Templates 2021 - Colorlib
You can the use the Jinja templating if the output is static and use ReactiveHTML to make parts of the page reactive.
If you take on the challenge please share your progress for inspiration.
I begin to use adminLTE, https://adminlte.io/. You can see I put some plots inside the divs, but it is time consuming to fully understand the structure of this kind of templates.
That is also my experience. That it can be quite overwhelming.