reducing the voltage clamped by a TVS diode for SPI bus
But this solution cannot reduce the clamp voltage which can be 10V~30V depending on the TVS diodes

So get one that has a lower clamp voltage!

You should also look at Zener diodes, or possibly clamping to the power rails with Schottky diodes.

Also consider dV/dt.  Whatever you use as a clamp can only react within some minimum time.  You have to make sure that the voltage doesn't get too high during that time.  One way is to add a little capacitance.  Consider the fastest the line has to settle to a new value, and make sure there are several RC time constants in that time.  For example, it takes three time constants to settle to 95% of the final value.

10 &Omega; seems rather low for the series resistance.  I haven't worked out what the time constant limits are given reasonable estimates of the parasitic capacitance, but my gut feel says 100 &Omega; is probably doable.  However, it's your job to do the calculations to make sure or to find a better resistance.