Amazing work, I am currently in the process of building something similar on my server for personal use, but yours looks really promising. Especially running sandboxes on your own can be tedious. Why have you opted for Modal instead of Firecracker or a similar micro VM solution?
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.