Alternatives to Google Autofill

Works on MicroG?

No...

Open Source Offline

Keepass

KeePassDroid is an implementation of the KeePass password safe for the Android platform. Keepass is available on all the major desktop platforms: on GNU/Linux KeePassX is a well-established client. Portable editions are available if you need to use it on a Windows machine.

KeePassDX

A light material design Keepass client to manage keys and passwords in secure database.

Open Source Sync

Bitwarden

Bitwarden is the easiest and safest way to store all of your logins and passwords while conveniently keeping them synced between all of your devices. Bitwarden stores all of your logins in an encrypted vault that syncs across all of your devices. Since it's fully encrypted before it ever leaves your device, only you have access to your data. You can host your server.

Available in the IzzyOnDroid repo

Lockwise

Open-source password manager from Mozilla

Closed Source Sync

It would be better to avoid them