That's looking better! Stretching is very important to be able to comfortably hold an aerodynamic position for a long time. I spent many years in Taekwondo, Muay Thai, and BJJ building up flexibility, which has served me well for cycling. I particularly like the hurdler's stretch:
(Note, back is flat and the person is reaching forward with both arms to . . . NOT with spine bent and head tipping forward.)
The quad stretch:
The split stretch:
(Again, try to reach as far forward as possible rather than lower your head to the ground to prevent bending your spine funny ways)
And the glute stretch:
Hold the stretches for 40 seconds, release for 10 seconds, repeat 3x per side going a bit lower each time. Stretch
after you workout before you cool down, not before a workout when you're cool and you'll benefit more from them. Generally speaking, the more flexible you are the lower you'll be able to comfortably get your front end, which means the less wind resistance you'll be fighting.
I personally think that the KOPS method is garbage (at least it doesn't work at all for me). First of all, the concept doesn't make any sense (your knee position is going to change relative to the pedal spindle when you go up or down even the slightest hill, and when you stand it's completely different too). It also doesn't really work if you have unusually long or short thighs. If you're average sized, it's not a terrible way to get roughly in a the right position . . . but feel free to keep tweaking if it doesn't feel perfect. For me, I need at least 40 km on a position change before I can decide if it's better or not.
Core strength is all well and good (I'd suggest heavy deadlifts and overhead barbell squats if you want to develop an incredibly strong core), but try playing around with bike fit first. If you're feeling too much weight on your hands, remember that sliding the saddle
back will reduce this. (Don't believe me? Try doing a squat with your shoulders forward. Notice that to keep your balance your ass has to move pretty far back? Now try doing the same without moving your ass backwards . . . you start falling forward. When your saddle is too far forward, it forces extra weight on your hands because your center of balance is moved forward. Also remember . . . because the seat tube is at an angle, every time you raise your saddle it it will move back a little. Every time you drop it it will move forward a little.