If no zoan was at their strongest when only partially transformed then they wouldn't use hybrid as their main fighting mode, period.
If Kaido went hybrid so he could 1v1 with his club, haki, monster stats in base..., then that's because, when we add all of that to the mix, the result of hybrid is above what both original and beast forms can achieve; therefore stronger.
Wonder how will Kaido fight 99% of his final confrontation with Luffy, when he sees the chance of losing and needs to go all out? Spoiler: not full beast, just like Lucci, Jabra, Kaku, Queen, Who's Who and so on. Any of these characters were or will be beaten while giving it all on hybrid because it's their strongest mode.
If Kaido went hybrid so he could 1v1 with his club, haki, monster stats in base..., then that's because, when we add all of that to the mix, the result of hybrid is above what both original and beast forms can achieve; therefore stronger.
Wonder how will Kaido fight 99% of his final confrontation with Luffy, when he sees the chance of losing and needs to go all out? Spoiler: not full beast, just like Lucci, Jabra, Kaku, Queen, Who's Who and so on. Any of these characters were or will be beaten while giving it all on hybrid because it's their strongest mode.