Molly is only available on Android, as far as differences it is a hardened fork of signal with an encrypted database, what that means in practice is even if someone was actively probing your phone to try to gain access to messages they wouldn't be able to due to the encryption. It's very useful if you are an active target or you don't trust your phone os to play nice. I personally use it myself and really like it but in general it's not terribly different.
It's only encrypted in a BFU state, (before first unlock). Police can probe your phone for data using a tool by cellebrite without root. GrapheneOS includes a auto rebooting feature to place it back in a BFU state but other phones will lack this feature. Using Molly's database lock allows you to not trust the OS itself by encrypting it.
So it should keep my database, chat history safely encrypted right?
Newer custom ROMs for my device have some issues since they moved on from 4.4 to 4.19 kernel and some other features, so system isn't encrypted at all.
The main issue with encrypting the database using Molly's setup is you'll miss notifications and calls until you unlock, this might be able to be fixed using a different database encryption setup but as it stands it would be inconvenient for many.
the more third party apps you have, the higher your attack surface and a decrease in security. I love my mods but this isn't really a solution and should be an os feature.
find my device would be more difficult to implement without a decently sized company backing it but a lock out function wouldn't be that difficult. it just needs to be implemented into the os itself rather than rely on the trustworthiness of a third party dev.