Dying over a permanent hair dye?

It will depend how dark the ash blonde dye is. If it is very light, then yes.
Since your hair is already blonde I don't think it is necessary though. If you want a cooler toned blonde you can tone what you have now (with non oxidative dyes) and save your hair the damage.

Is it better to use wella t18 toner instead of a permanent hair dye to give it a cooler side? I did a spot bleaching today for dark bits and my hair is very close to banana shade (inside). Of course I will have to wait. I already bought a coconut oil and I have to do 2 treatments first. The hair looks fine, but I just want to avoid a damage. It's never too early to start caring for hair until the damage starts showing 😂😂😂

Generally, we just don't really like toners you have to use peroxide with, there's not really any need and it causes extra damage.

Tone with direct dyes or silver shampoo. If your hair is inside of banana yellow, it's about right.

If you are still planning on going silver there is no need to wait, you can use Pravana silver now.
Those wella toners work in exactly the same way as permanent blonde dye, they are best avoided if you want to avoid damage.

ok fine, you both convinced me 😂 I'll use pravana directly on whatever I have now... But I was planing to do a keratin treatment (since I don't wont to keep straightening my curly hair for the whole summer. Do you think I should dye after doing keratin? I heard keratin strips dye from hair 😂