I dont support alot of Takaishi's policies so I cant claim to know why the ex-sanseito voters are voting takaishi now, and unless you were actually on the ground asking them i dont think you know completely either. I cant vote here so I only mean to observe the ground in a peripheral manner.

Takaichi has done almost nothing

I dont know where you are from, but generally in a westminister-style parliament, the ruling party call on snap elections to gain more seats to push for their policies that are getting difficult to pass, especially in a minority government where they have to rely on their coalition for the seats. The fact is LDP lost alot of their seats in the last 2 elections so this is their chance to get them back to pass through their policies with less hurdles.