If the handwriting is too much and too dense, there is no way to save it, but to wash it off. I suggest you stop wearing this dress, or only wear it under special circumstances. After all, you have to wash it when it is dirty, but you can't leave your handwriting on it when washing clothes unless you use an indelible marker or other more durable pen with special ink.
Nowadays, more and more people choose to write their names or wishes on their clothes when remembering. Some people like to have their idol stars sign their clothes, but obviously such clothes can't be worn too many times. After all, if they are dirty, they must be washed. But handwriting, if it is not written with a special pen, will disappear if it is not washed twice, and your idol may not have the chance to meet you for the second time, that is to say.
Therefore, I suggest that you don't write on your clothes when you think about it in the future. If you want to write, you should also write on special clothes. This dress will not be worn again or can be worn rarely, such as school uniforms. You can write your signature or blessing on the school uniform when you graduate, and you will not wear the school uniform in the future, so the handwriting on this dress can be kept for a long time. If there are special circumstances, you can wear this dress again, and then you can put it away after wearing it only a few times. This is much better than writing it on ordinary clothes and then putting it on and washing it constantly.
So if you really want to wash it, wash it selectively, piece by piece, and avoid the handwriting on it.