Ok, #1 you're not FL sizing. If you were there would be no question about any partial length sizing, because you would be
full length sizing. What You're doing is body-bushing sizing.
As far as tune, seating and primer selection are not tuning, but prerequisite to good tuning.
Powder can be used for powder node tuning, or barrel tuning, rarely both. Pick one -best accuracy, -or forgiving load.
Neck tension adjustment doesn't always offer improvements, but it can where you need more powder burning in the chamber instead of X-distance down the bore(with a tension increase). It can lower tension variances (with a tension reduction). The adjustment is nice in that it can help you be sure to never size more neck length than seated bullet bearing.
With this cartridge, after load development at very near full seated bearing length worth of neck length sizing, I backed off sizing length while group shooting. I came to this reduction before MV started changing, so this is where I left the die setting.
I wasn't tuning, but setting minimum tension for the chosen load, to keep tension variance as low as possible.
It's a turned neck, and thicker or thinner necks would have led to a different setting.
Maybe you could tune with neck tension. Maybe you could affect SD.
But if relying on this for tune, your [spring back force x area gripped] better be super consistent. I think it would need to be measured, and right now there is no way to actually do that.
Given this, It's easier for sure to rely on the right powder charge for tune.