I already have my own back end, but sveltekit is all about using their back end stuff. And their back end is 100% all in on serverless functions. If you want to write a more traditional back end and do something like grab private API keys when your service starts up, well sorry no easy way to do that, although there is an active git issue thread with people pretty much begging for the functionality.
Doing stuff like initializing logging libraries and passing them to models, or fetching API keys from secure storage buckets, is such a very common operation in any at-scale app, that I am beyond obscenely surprised sveletekit doesn't offer an out of the box way to do those things.
I eventually beat it into shape of generating a true SPA, generating paths that weren't at the root of the domain, and I convinced express to serve up assets accordingly. Took way too many days to get it working though.
A key strength for Rails is Ruby and a key weakness for any aspiring JS equivalent is JS and the JS ecosystem.
If the Rails team had had to spend time adapting to 6 different alternatives to bundler, rack, and every other library they use it wouldn't be what it is today.
Oh, and also they provide a repl to your app code and data… that is huge and super hard to find in the js world
I'm very bullish on SvelteKit — hits all the points you mention above and outputs a minimal, performant full-stack app. https://kit.svelte.dev/ Still in beta and changing a lot but really really lovely to use.
Heck, to scratch my own itch I'm even making a Rails-like SaaS boilerplate to go on top of SvelteKit to give me all models, user auth, admin dashboards, payments, etc that you need in almost every app these days: https://sveltesaas.com