Often enabling SMS 2FA also enables SMS as a password reset option, which does make you less secure, since SIMjacking is easier than guessing a reasonable not-reused password.

If that is the case though, you are not really enabling SMS 2FA. It is just SMS (1F) authentication.

Real 2FA would (theoretically) never make your account less secure than 1FA, because even if the second factor has 0 security, it shouldn't decrease the security of the first factor.

However, it is true that this may not always be the case for imperfect implementations, like your example. I can aldo imagine that social engineering might have a higher succes ratio if the intruder can say "it really is me! I have the correct second factor, I just lost my first factor...".

With Google you can disable SMS-based account recovery independent of your 2FA options.

I think Google actually allowed you to just use your phone number to recover your account, so it was effective 1FA. In that case it would be less secure. They might have changed it now though.