It's not a proposed law it is a new law. It was signed by the governor. It will start in 2028. (For taxes filed in 2029)
Whether I like it or not is not really the point. The point is it is unconstitutional. If the state wants to impose an income tax it must do so by amending the Constitution not ignoring it.
The voters of Washington have stated clearly that they do not want an income tax.
Income tax, as an example the federal income tax is actually very regressive. People pretend it's a progressive tax but it's not. The wealthy are able to avoid the taxes.
I don't really want any income to be taxed. I prefer to tax consumption rather than production.
As far as what I would do instead I don't know. Personally I think Washington's budget suffers more from a spending problem than a revenue problem. I would try to start there.
What would I do if tasked to figure out new revenue?
I would try to encourage and help start new businesses. Especially small to medium size business increasing the tax base.
I would tax all business profit equally instead of giving huge breaks to some companies at the expense of others.
I would raise non-resident fees for State Parks, Ferries, and some other servises.
I would eliminate the non-resident sales tax exemption. (This allows residents of some states to get a refund for some of the sales tax paid in Washington)
I would look to see where I could increase service fees. In general, and when possible a government function should be mostly funded by the people or businesses using that function. For example, the liquor control board should be mostly funded by the businesses that sell liquor. Restaurant inspectors should mostly be paid for by the restaurants that they inspect and/or their customers. The ferry system should be largely paid for by the people who ride the ferry etc.
Edit:
The State Constitution definition of property in the taxation section: "The word "property" as used herein shall mean and include everything, whether tangible or intangible, subject to ownership"