@pray_daily Again, this is coming from someone who has a much much uglier v-sit, and who can only tuck planche. But I'll try to be useful:
V-sit
For V-sit, you can kind of approach it in multiple ways. Probably the best is too continue improving hamstring flexibility, so that he can bring his legs closer to vertical without having to bend at the knees. An alternative -- to an extent -- is to strengthen the abs so that you can pull your legs closer to your chest. Think of your hamstrings as a spring, where the more acute your hip angle, the stronger the restoring force becomes. Increasing flexibility is like reducing the spring coefficient, whereas increasing core strength is like having a larger external force that can displace the spring farther from equilibrium. Humans aren't actually ideal spring systems though, so this only works within a small range before you have to worry about injury risk.
The other way he can get his legs more vertical is to simply lean his torso further back. For this, you don't have to bend any more sharply at the hips, but the catch is that it takes a non-negligible amount of rear delt/mid back strength to hold that position and not fall on your ass. You can think of this approach as a kind of progression towards a reverse planche, which is obviously very difficult.
The correct route forward is probably some combination of all 3 of these approaches, and OP just needs experiment to identify which of these areas has the most room for improvement
Planche
The arms look fully locked out, which is good to see. Obviously make sure you're not progressing faster than your tendons can adapt. If I had to guess, i would say that the shoulders and biceps are probably the limiting factors, just because that's almost always the case. To confirm this, OP should try doing a reverse hyperextension with his trunk (lay your torso down on e.g. a table, gripping it tightly, and try to lift your legs behind you so that your thighs and shins are horizontal), just to make sure his lower back isn't the weak link.
I'll emphasize once again that I'm not an authority on the subject, but I think OP could probably benefit from focusing on hypertrophy work for his arms and shoulders for a few months (?). That could mean going back to full tuck planche for longer holds, but it could also mean doing bicep curls and frontal dumbbell raises (arms locked out of course. Bonus points for palms facing forward, but progress safely with it), if op has access to equipment. I think if OP packs on more muscle mass first, then later on he should have an easier time training it to be strong in the planche position specifically.
Hopefully at least some of that was helpful!
!