Black probably wins Connect6 on 19x19 board

@eeoo figured out black has advantages in every openings several months ago. To verify whether black wins on 19x19, I started a new Connect6 KataGo training in August.
My result until now is that black's winrate is 80%, and the optimal game length (both player play perfectly, black tries to win fast and white tries to lose slowly) is about 120 stones (60 moves, 30 turns per player).
Here are some strong defences for white.

For larger boards (21x21 or larger), black wins much easier. Only about 80 stones.

I will collaborate with @dblue (the author of Rapfi) to develop an NNUE-like engine, and try to make the result more rigorous and detailed.
KataGo for Connect6 is opensourced: https://github.com/hzyhhzy/KataGo/tree/ConnectSix2024
I will release a package after training finished.
Old package (20220517): https://github.com/hzyhhzy/KataGo/releases/tag/20240406

Now the longest win is reduced to 109 stones

for strong defenses you mean these are some of the best possible plays by white?
Also how come with a larger board black wins more easily if already the longes wins take only 109 stones? I mean what influence has the edge if it's not even reached?
Thanks!

yes, best possible plays by white
For larger boards ~80 stones. Obviously you can't see the the influence of the edge in the above images because they are just at the beginning.

One very sure result for Gomoku is that, black needs 33 moves to win on 15x15, but 31 moves on 19x19
For Renju, 35 moves to win on 15x15, 33 moves on 19x19
Connect6 is much more balanced, and needs much bigger board and much more stones to win

so do you think there should be an opening rule with restrictions on black as in Renju & Gomoku?

One thing that would be very interesting to understand is the added power of MCTS vs "pure intuition" (network eval without lookahead) for your bots. I tend to think of connect-k games as needing a lot of logical search, but then I never developed much intuition.

In Abstract Play, there is a swap-3rd opening rule variant that's proposed by eeoo here: https://littlegolem.net/jsp/forum/topic2.jsp?forum=70&topic=191

It's supposed to make the games more balanced and give it more opening variety, but unlike Gomoku where the first person advantage is quite drastic, I think it requires players to be at a certain skill level to appreciate it in Connect6 because it's overall still quite balanced for most people.