I have some questions and feature requests, most of these are about backups. Before I continue I want to quickly express my gratitude for the GrapheneOS team, it's remarkable software and I don't know what I'd do without it. I also apologise if anything here has already been addressed elsewhere, though I have done a quick search for similar issues on this forum and I'm happy to be referred to another thread if it answers a question. I have indeed read the docs, but I only did so when I installed it for the first time which was in 2022, I don't have time to always keep up to date and my memory isn't perfect.

Questions

What is the best way to test backups? Is it safe to recover the backup on a separate Profile? Will this mess with any data on other profiles? What if I enable D2D (which I don't)?.
Despite getting a notification that my backup completed, some apps show that they are still being backed up, and SMS text messages in particular never seems to finish. If I backup to external USB, I need to know when the backup is actually completed so that I can remove my USB, lest it incur data loss.
Is there any difference between copying the .SeedVaultAndroidBackup folder to my PC, vs backing up to USB using SeedVault? Will the content be the same? Personally I would rather the former, because (a) backups can be done automatically without inserting a drive and (b) it's faster because I don't have to wait for the backup to be finished before removing my drive, and (c) it's more reliable, no chance of data loss due to removing drive before backup is completed (see 2), but of course if something didn't back up properly I'd still have the same problem.
When choosing "App backup" in Backup settings (ie. saving the apps themselves), could this ever include user data? Is there a reason to choose this besides just being faster to recover?
Is D2D just App Backup but includes apps that aren't normally backed up?
Are backups using SeedVault reliable, given that they appear to be experimental? What are the known limitations/issues? Other threads on this forum suggest it's unreliable but there's conflicting information. In what way is it unreliable?
Are directories such as Music, Podcasts, Movies intended for app data or for users? Is it safe to use those directories myself?
What is Settings > Passwords & Accounts? I really have no clue what this is. Under Passwords, passkeys, and data services, selecting an app such as Firefox brings up a prompt which says "New passwords, passkeys, and other info will be saved here from now on. Firefox may use what's on your screen to determine what can be autofilled". Is this primarily for form autofill? How does it affect the handling of hardware security keys (eg. Yubikey)?
(not GrapheneOS specific): Will ext4 format ever be supported on Android or GOS? Are you limited to only FAT and NTFS?

Feature requests

I'm putting these here mostly for feedback and discussion, they're probably duplicates anyway.

It would be nice to have a progress/info bar that shows what exactly is being backed up, because I'll be staring at the status page cluelessly waiting for GmsCompatConfig to backup (why is this taking so long?), some other apps that shouldn't be taking so long, one app has a circular red exclamation icon (despite enabling "unlimited app quota", this app might've failed because it has about 50GB of downloaded content, so is "unlimited" really true?) and SMS text messages which never finishes (I don't have that many). Some transparency would be ideal here, clicking on the icons should reveal some info about their progress and why they supposedly failed. I also don't know if there's a bottleneck with my USB drive since I can't see MB/s.
I should be able to retry backups for individual apps that failed to backup, so that I don't have to run another full backup. Simply clicking on it on the Status page should suffice.
Option to export SMS and call history. We can already export contacts to .vcf in the Contacts app, so it's odd that we can't also export SMS and call history. The reason I deem this important is because I don't want to have to rely on the SeedVault backups, because SMS messages never finish backing up so I'd consider it unreliable or maybe even corrupted. Contacts and call history appear to backup fine but manual export should still be an option.