And the users might have a problem with it too: if the phone comes preloaded with every app that any market sector might want to use {...} and if I can't install weird niche stuff
The way this class of problem has been solved in the past on Linux phones is by trying to handle accounts with a standardised API.
Palm/HP's WebOS had the very advanced Synergy, and Jolla's SailfishOS has a simplified version as Accounts.
It's these system's job to handle logging into servers on one side (Google Account, etc.), and exposing standard APIs to apps on the phone on the other side (mail, contact list, messages, upload of photos, etc.) Phone used to come with a set of standard account plugin (Google, Facebook back when they used to have an API, Microsoft Exchange for business settings), and a couple of standard apps (Mail, Camera, Chat, etc.).
The user can install additional plugins to handle additional type of accounts (e.g. anything with a libpurple and/or telepathy plugin can be added as a chat provider to the system chat app).
The main problem of this approach is that most online platforms have aggressively moved away from having open API and on locking you to only be able to perform actions from whithin their APP (e.g.: Facebook shut down their XMPP access, you MUST chat only exclusively from their Messenger app; Slack doesn't expose anything useful), and Android is geared heavily toward this type of interaction (the camera app doesn't handle "Upload to an arbitrary account", instead it opens a list of apps which can use JPEGs).
Efforts like EU's Digital Market Act might help tip the tendency back toward more open platforms.