The concrete bug in your code: @changesets/config doesn't export anything named read, only readConfig. Your import import {read as readConfig} from '@changesets/config' is aliasing a export that doesn't exist, which means whatever runs at readConfig(cwd) isn't the real function. That's likely from an older revision of the assemble-release-plan README, the current one imports it directly with no aliasing:

import assembleReleasePlan from "@changesets/assemble-release-plan";
import { readChangesets } from "@changesets/read";
import { readConfig } from "@changesets/config";
import { getPackages } from "@manypkg/get-packages";
import { readPreState } from "@changesets/pre";

const packages = await getPackages(cwd);
const preState = await readPreState(cwd);
const { config } = await readConfig(cwd, packages);
const changesets = await readChangesets(cwd, sinceRef);

const releasePlan = assembleReleasePlan(changesets, packages, config, preState);

Note readConfig also takes packages as its second argument (it'll fetch them itself if you omit it, but passing them avoids a second getPackages call), and readConfig returns { config, errors }, not the config object directly, so const config = await readConfig(cwd) in your version was also assigning the wrong shape even before the import issue.

That said, for this exact use case (get a release plan without wiring these five packages by hand) there's a purpose-built package that does all of the above in one call: @changesets/get-release-plan.

import { getReleasePlan } from "@changesets/get-release-plan";

const releasePlan = await getReleasePlan(cwd, sinceRef);
console.log(releasePlan);

Its own package README says as much: assemble-release-plan exists mainly for testing and CI cases where you already have the pieces in hand, for most consumers get-release-plan is what you want, it removes exactly the kind of wiring mismatch you hit.

as the other reply says, most users should use get-release-plan instead as it does most of the complicated logic for you.

if you still need help, feel free to open the discussion again!