I have also been waiting on the SR model and now that it is finally available been tracking the Legislature to try to time the purchase. I am definitely not an expert reading the bills but unfortunately I think the sales tax break is going to be after June 30th which means it won't be available until after the Federal Tax Credit gets cut in half again.
Here is text I found at the top of page five of the latest version of the bill which was recommended this week to be passed by the Senate Transportation Committee. It is called the second substitute bill.
(e) "Qualification period start date" means the first day of the first calendar month after June 2019 that is at least sixty days after the effective date of this section.
http://lawfilesext.leg.wa.gov/biennium/2019-20/Pdf/Bills/Senate Bills/5336-S2.pdf#page=1
I think that clearly means it cannot start until after July 1st and could be later as there has to be at least sixy days after the bill becomes effective.
Page 2 has a line that clearly says the sales tax break becomes available after the qualification period start date.
(1) Beginning with sales made or lease agreements signed on or after the qualification period start date:
At this point it looks like it is still capped at $1000 which is less than the drop in Federal Tax Credit so it looks like taking delivery prior to June 30th will be better for anyone who will qualify for the entire $3750 Federal Tax Credit.
Of course there is always the possibility that the Federal Tax Credit gets changed (removed or restored) but the last I saw, any attempts at change have not made any progress.