I’m wondering if it isn’t better to just whitelist cookies for the sites I need to log into and not bother with a password manager extension (keepasxc or bitwarden). I try to keep the number of extensions in my browser to a minimum to lower the attack surface. And why involve one more entity in the password story? Are there any problems with using the (1st party) cookies of sites I have signed up to and use to keep me signed in?
No, one and two are not the same. Your PM is not unlocked unless you enter that master password and it will lock after some time period. Many common PMs sync too. Plus you should be using 2FA. If you care about security avoid auto login.