I have thousands of passwords in my Keychain. I don't want to transfer them to LastPass (even if that is possible) but LastPass seems to have taken over from Keychain and I can find no option to use both.
Hello,
While you can select both iCloud Keychain and LastPass in your phone's Settings app > Passwords > Autofill this isn't a seamless experience--Keychain will generally show first/suggest autofill for apps or sites that you log in to, but to access LastPass you should be able to click the key icon at the top right of the keyboard and click "LastPass..." then select credentials from your Vault. Only selecting 1 autofill service in Settings > Passwords > Autofill will eliminate the extra step of having to click the key, select LastPass, etc. but you can select both to have both options available in Safari.