Fintft
G.O.A.T.
To hit flat and through the ball, there are four adjustments that you'll have to make to your form:
1) Change to an eastern or modified eastern (halfway to SW) grip.
2) W/ an eastern grip, you will need almost a straight take back. I'd use a narrow loop though, to keep the movement going. For a mod eastern grip, use a little more open loop (see Roger Federer clips.)
3) Closed to neuteral footwork. Re-asess how close you will need to get to the ball.
4) This step should take care of itself naturally. Use less windshield-wiper and finish over the shoulder.
About your 4th point, this is something a head coach at my club pointed to me, a couple of years ago when he first saw me...He understood that I wanted to generate higher swing speed, but was adamant of me trying to go first through the ball more and only afterwards coming over the ball...
He wasn't even sure if one should have a fully straightened arm at contact (although I think that it helps what the OP wants to achieve).
You know what other minor factor might help imho?
Using a heavier frame, i.e. I was watching closely (a while ago) someone with a K90 hitting against something with a very light frame and I think that the momentum alone of the K90 helped carry it more through the ball….