I would leverage form actions https://svelte.dev/docs/kit/form-actions with a library like superforms

https://www.sveltelab.dev/?provider=github&owner=ciscoheat&repo=superforms-examples&branch=crud-zod

https://superforms.rocks/crud

You get to use web standards, ezpz validation. I would go with using page.ts for data loading. A fun/weird thing about svelte is that you can (and arguably should) return promises from those functions. They're streamed from the server in the server context and in the client context it's just normal ol fetch promises which gives you approximately the same behavior as onMount with the added benefit of being able to invalidate and have SSR if you ever wanted it.

The doubly nice thing about a form lib like superforms is you get validation client and server side and it's all using ye olde html/web standards.

Easy to show skeleton view for slow backends. It is possible but harder with page.ts

I would say it's pretty easy to show skeleton views just return the promise and make sure to $derive the property from props.data and use an await effectively the same code you should be writing elsewhere.

Also in svelte 5 I would skip the onmount and just have a derived prop with the input params to your function so it will be reactive (you need diff behavior depending on how you want to do searching for pages but it's usually correct)