What is a long term solution for this? Is there something more than "build tanks that don't leak"? I'm sure spaceX has top design and materials experts, now what ;-).

I think its likely not the tanks but rather the plumbing to engines and the engines themselves leaking (sense lines, etc).

Next engine revision (Raptor 3) should help, as it is much simplified and quite less likely to leak or get damaged during flight.