yes, you can connect them and you can import from last.fm. I was in the same situation as you, first I had both simultaneously running for some time, because I needed to get comfortable with the idea of removing last.fm. I also have data since 2008 so I felt a bit insecure ‘risking’ that. But after a while I concluded there was really no need for me to keep last.fm so I removed it. Haven’t had any regrets. ListenBrainz isn’t perfect but, despite it’s small development team, it’s sgnificantly improving every year.

https://listenbrainz.org/settings/music-services/details/ Here you can “Connect to your Last.FM account to import your entire listening history and automatically add your new scrobbles to ListenBrainz.”