You're over-simplifying panthers role. While it can start combat with invisibility for a surprise round, that is hardly the entirety of its role.
Except... It is.
That's the literal only thing the form has over any other form. The invisibility.
Literally, it has 4 abilities. 1 is the regular attack that all (Non-cat/birb) forms have. 1 is a mobility action that all (Non-bear) forms have (And several of them also apply prone, to varying degrees of success). 1 is a conditional attack dealing bonus damage that some forms have their own variant on (The wolf has the "Make an ally crit" thing for example). 1 is the invisibility.
The only unique thing in the form, is the invisibility. But given that invisibility is mostly useful for its ease of creating surprise rounds (Or simply sneaking past hostile enemies but this feature is nigh useless in BG3 not only because of the party, but because you want to kill everything for exp), that's the niche that the form fills. You use this form when you want that specific thing (Especially since you get it at the same time as Owlbear so it's not like Wolf where you only use it because you don't have anything better)
And comparing it to bear is just not a good argument. Bear is probably the worse form in the entire game, if you don't count critters/badger. Goading roar doesn't actually "taunt" anything. It just applies disadvantage on attack rolls against your allies to effected enemies.
It's a perfectly fine argument.
Bear has a unique skill. Goading Roar. If you have access to multiple forms, that would be the only reason to utilize the form.
The fact that Bear sucks with it's paltry AC, lack of mobility skill and overall mediocre effect of the unique ability doesn't change the fact that when you have a plethora of forms, the reason to use Bear is its unique skill. Just like with all the forms available, the reason to use Panther is its unique invisibility. Or the reason to use Spider is to use its unique ability in Web.
That's simply the general way that having multiple forms is normally implemented in games (TT or video). The idea is normally you swap forms to suit the situation and leverage what they provide. Rather than the singular form restrictions where there's more pressure put on to make all options be multi-faceted.
While I'm not opposed to the idea of single form restricted Druid, so each form you can pick is more well rounded but provides its own take on things. Though that would be more of a subclass type change rather than a simple homebrew to Druids overall.
