Then maybe look at https://lit.dev/ it might be perfect solution for you, stencil i've heard is also nice. Since its custom-elements you just add custom tags and they will self upgrade.
Lit elements looks nice and it's good to see first-class support for WebComponents. I'm just a bit concerned about having to write declarative HTML in JavaScript functions. Mixing declarative and imperative code can lead to some unwieldy templates so I prefer am approach with minimal templating enhancements sprinkled into standard HTML. What would be an approach, similar to Vue, to enforce the separation of imperative business frim declarative layout definitions using LitElement, Polymer, or similar WebComponents frameworks?