Data loading issue with unconventional (?) hybrid SPA/Node setup

[wont-work]

First of all, let me clarify what I want to do just to make sure no XY problems creep in: I want to make an app that can be built both as an SPA and Node server.

The Node version will connect to a pre-existing backend anonymously (serverside) and display data that way, with an option to log in as a user and get access to user specific data.

The SPA version will NOT have that anonymous option and will always require logging in, with the ability to select different backend hosts.

Both to make my job easier and to keep any user credentials far away from a potentially untrusted Node server (which isn't a realistic threat model but I'm sure some privacy nut out there would complain, considering the audience I'm building my app for) all authorized communication will be done in browser, with all user tokens being kept in browser, so any shortcomings of having loading screens and whatnot are not an issue for my use case.

---

But I've seem to hit a little snag in making this work: I'm not sure about which combination of SSR and prerendering options I need to pull this off correctly. Right now, my root +layout.ts has:

export const ssr = true;
export const prerender = true;
export const trailingSlash = "always";

And my (app) root layout (which is where most of the app routes will be in) has:

import { UI_BUILD_MODE } from "$env/static/public";

export const prerender = UI_BUILD_MODE == "spa";
export const ssr = UI_BUILD_MODE == "node";

When I build as Node, everything's fine, but when I build as an SPA, it returns a "404 undefined" error UNTIL I try to click a link pointing to / which will then display the page the way I expect it to.

In the dev tools, I can see two 404'd fetches to http://0.0.0.0:8080/__data.json?x-sveltekit-invalidated=1 (and 100), and a console.log I put on there says the root layout has a data of, unsurprisingly, null (as there is no server to execute +layout.server.ts). The weird thing is, another console.log I put on the (app) root layout doesn't get triggered until I navigate to /. So whatever my issue is, it's happening at the very top +layout.server.ts, which for now reads a few cookies and doesn't do anything else. (And of course, I do check if LayoutData is null in the component itself.)

Does anyone here have any ideas on what may be causing this issue? I'm also attaching my SvelteKit config, just in case my error lies there. I haven't modified the Vite config in a way that seems relevant to this

svelte.config.js.txt

Thanks in advance!

Edit: Here are my (pruned to just the relevant ones) devDependencies just in case:

	"devDependencies": {
		"@sveltejs/adapter-node": "^1.2.4",
		"@sveltejs/adapter-static": "^2.0.2",
		"@sveltejs/kit": "^1.5.0",
		"svelte": "^3.54.0",
		"svelte-check": "^3.0.1",
		"vite": "^4.3.0"
	},