In my experience iCloud Passwords is a superior cross-platform password manager than LastPass today, so I’d recommend going in the other direction. (Source: I switched from LastPass to Passwords a couple of years ago.)
In Coders at Work, Peter Seibel quotes Jamie Zawinski as saying that certain Netscape acquirees were people for whom "cross-platform means both Windows 3.1 and Windows 95."
You might not have heard but Lastpass got hacked fairly recently. I would be careful about storing your passwords in the cloud (or at least that cloud). Keepass / KeepassDX would be safer. Sync devices with SyncThing.
Disable iOS passwords and set lastpass as your only password autofill source. It’s far from difficult. In settings under autofill passwords