Interesting I'd still go for Lit (https://lit.dev/) though if you want something light weight and no/little build.
Your reply encouraged me to experiment a little bit.
Turns out, CalDOM already seamlessly merge with native Web Components. I just didn't see it previously. :(
It can be used to create reactive, stateful & native Web Components. Check out this example: https://caldom.org/#web-components
If you happen to play with it, please let me know your suggestions to improve it.
Thanks!
In my opinion, this is the future. Especially, Native Web Components will take over in this decade. CalDOM's long-term plan is to seamlessly merge with native web components when it's widely supported.