On Phasma’s return, let’s put it this way: The issue is definitely that she does little in TFA, and appears to die, then returns in TLJ to do very little again then appears to die again.

Removing her from TLJ is one way to minimise some of the relative weakness of that plot. Her arc, or rather Finn’s arc, becomes that she’s Finn’s overseer in TFA and ultimately he disposes of her. We don’t see her again.

However, without needing to wait two years between movies (which we can do now that TLJ is out), I think the better arc is as follows:

She’s Finn’s overseer in TFA
Finn leaves the First Order
She doesn’t appear again in TFA - it’s not implied that she died on SKB
Finn’s a traitor to the FO but struggles to commit to the Resistance in TLJ
She appears in TLJ the way she currently does, reinforcing that Finn is a traitor to the FO in a scene where, through DJ, we learn one of the key issues of not picking a side.
Finn kills her for good, shortly before we see him properly commit to the Resistance.

That way, one of the key weaknesses of her character (“die” > return > die) is removed, but she gives Finn the stronger arc and his (one on-screen) killing of her is a part of his journey towards being able to commit to the Resistance. The change this’d require is her removal from the SKB scenes of TFA.