Yes, Saki's main hand is her ridiculous attack speed (and ranged assassinate that gets to proc fairly often thanks to that speed), so she is more of a wave thinner to be used versus unarmored foes whom she shreds. It will later be reinforced by her Skill Awakening, that will let her attack up to 3 (three) targets in the range at the same time.
Horace on the other hand suffers from the massive slowdown upon awakening, but offsets it with equally massive attack increase. This makes her more of a duelist / duelist supporter, especially against armored foes in skill.

Memento's usefulness is equally proportional to the amount of units you'd rather keep out of your hair for a time and, especially, urgency of keeping their attention off your team. So yes, she does not offer much outside of a few delicate situations, but in these situations she can be invaluable.

Bashira brings quite a good burst on the table. Rita, on the other hand, only offers utility of stealth - something nice to have in a few situations, but completely superfluous in most. So right now you will see much much more use from raising Bashira, especially when you have low-cost deployment covered by your Spica. Eventually you might want to raise Rita as well, but she is not a priority.

Did I spy Marius there? And untouched at it? Well, if you ever find yourself wanting of a more magical AoE (and you want to have at least two good AoE running), you have your answer right there! He outranges Mehlis out of a skill and outdamages her cleanly, and, if push comes to stove, he can contribute his considerable power to healing)

You'd also see a good use out of raising both your vanguard strategist (I mean, you have Uzume! While she's not as fighty as Shuka, her AW passive is just drool-worthy)) and rearguard tactician (because who don't want to spam skills more?)). Maybe Len's not that much of a priority, but don't neglect her much)