For my power button problems I'm now considering having a hard on-off switch between the powerboost and the raspi so I can charge the battery while the raspi is off (this would also be used as 'on') and combine this with a soft shutdown tactile button wired to GPIOs for quick and easy shutdown for when the battery level is getting dangerously low.
On the one hand is feels a bit stupid to have two power buttons but on the other hand it feels like the most straightforward solution nor requiring a lot of precious GPIO pins, or at least for now.