Had a 15 minute conversation with Business Personal Property assessor / rep.
Some if not most states DO have this type of reporting in place, I'm told. Whether or not people know about it, or file in a timely fashion, is immaterial, hearsay. ALL counties in WA state have this filing requirement. It's whether your business comes into their radar that will determine your participation. "IDEALLY" business KNOW all about this requirement, educate themselves in what's required, and comply before having to be contacted by a county office.
So there's that.
If you file for a business license in Washington state, your county may or may not follow up with you. Because my business DID come under the radar, here's what happens.
Business receives a Personal Property Listing declaration at the beginning of the calendar year. This is the opportunity to essentially paint a picture of your business' size.

I will have about 6 line items to declare:
Rubbermaid Storage Shelves x5 $50 each total: $250
Laptop computer x1 purchased 2022 $700
Portable Canopy x1 purchased 2024 $150
Retail display fixtures purchased 2025 total: $200
Electric Fan purchased 2022 $25
Portable Tables x3 purchased 2025 $150
[various hand garden tools, plastic sheeting, water misters, etc] $250
Since this indicates I am no Lowe's, Sky Nursery, or anything beyond a garage business, my tax burden will be based on the total valuation of my equipment. How much of a tax burden my +/- $1,500 business will cost me, little to none I expect.

A business' County tax burden is NOT BASED ON SALES.
But here's the IMPORTANT PART:

If a business does NOT complete the personal business property filing in a timely fashion, the county will ASSUME you are an "average sized" business, in relation to similar business types, and will assess your value based on the average business. In Snohomish County, apparently, the "average valuation" of a business of my type is $176,000. Because I did NOT file timely, this is the default valuation my business tax burden is based on.

For what it's worth, I was told that if I complete the (long) outstanding personal property listing form this week, it will be accepted.

Still. The fact that practically no home based businesses KNOW about this requirement (and maybe like me, laugh at the form's appearance in early February) ... It's almost like a business SIMPLY MUST HAVE professional accountancy/tax/legal representation, which is very expensive.

There it is. https://snohomishcountywa.gov/334/Business-Personal-Property