I'll admit to not having read the manga... and having only vague recollections of watching the series when I was younger. Certainly could be darker than I recall.
The manga was considerably darker yes. They... made it a lot more friendly in the anime.
The monsters from the manga are nightmarish eldritch abominations.
As far as I'm aware, which amounts to a "I
think", Naoko meant for it to end after the fight with Beryl. Before Silver Crystal shenanigans. As in it ends with literally everyone involved dead.
Usagi is also... optimistic is a Realist way. She doesn't think everything is going to end well, instead she always looks to the Silver Lining. She's a "Devil in the Details" very different, but still very similar to anime!Usagi.
She's also legitimately very smart. She just doesn't want to work because she's aware on some level that she's the Princess, so she lazes about instead.
PMMM was a... overreaction return to roots. It got back to the basic darkness, but took it too far really.
I am glad it came along though. The Magical Girl genre was getting too... saccharine really.
Tokyo Mew Mew could have been great, it was slated to be eldritch horror in the same way Sailor Moon was, but the editors just mauled the poor thing.
Not that light-hearted is a bad thing. I just feel you need a good amount of both of the sub-genres. Honestly the CCS reboot/continuation is probably appreciated all the more because the darker sub-genre made a come back as well.