Xen on Gentoo with Gentoo VMs. I've scripted the provisioning in bash, it's fairly straightforward - create lvm volume, extract latest root, tell xen whick kernel to boot.
Ideally would like to netboot a readonly root off nfs and apply config from some source. Probably bash :D
Some things like opnsense are much more handcrafted because they're a kind of unicorn compared to the rest of the stuff.