I'm actually starting to think this might be the way to go, with one caveat: TWF should be a reasonable option without any feats/themes, just like sword and board already is. (Maybe -3/-3?) You'll notice you don't need the Guardian theme to use a sword and shield; it just makes that style more effective.
Yup. You just make bare-bones / anyone can use TWF balanced against the other baseline attack modes (sword 'n board, great weapon), then use the Themes to give the extra 'oomph' for specializing in TWF. No reason to cordon off the attack mode unnecessarily.
Personally, I'd also like to see 'single weapon w/ open off-hand' attack mode have a theme attached to it as well, like the duelist theme. Perhaps with an introductory feat that gives something along the lines that if wielding a Finesse weapon in one hand with nothing in the other, if you have Advantage on attack your weapon has the Brutal property, or you crit on 19-20 (the bigger crit range here wouldn't bother me, because the Finesse weapons are all have lower damage dice.)