We are using next.js for the admin interface of the app. Works great with Polaris and AppBridge. Svelte is only used for the widget we are providing to the merchants.
I share your view that Svelte is a great framework for small projects. It is also really useful if you want to build small standalone javascript widgets. I've used it at work to build a fast video testimonial slider for Shopify [1]. I really liked the component format as we could quickly integrate our code in the existing HTML layout.
The Svelte built-in store has also been useful for us to manage the current state of the application. Overall development with Svelte has been really quick.
Just noticed your website checker might have a bug: https://onlineornot.com/website-down-checker?requestId=Kfd51...