Hi there,
It sounds like the Hubspot integration available on the App Store isn’t enough, and you’re looking to build your own.
If so, developing a plugin will definitely be your best bet. We haven’t tried using Retool (would be interesting if anyone in the community has) to build a plugin, but looking at their website, they work with Javascript, so that should be compatible with the Plugin SDK.
You can also try using our Front UI Kit to help save you time designing the components. This is what we normally recommend to developers who will be building with React/JavaScript.
Hope that helps get you started. Please post any and all other questions you have as you build out your integration!