I can download an Excel spreadsheet from my state Department of Revenue with the tax rates for every jurisdiction (358 in total, paid per the point of delivery, not the point of sale, Washington State). Anyway, I would like to be able to simply import the rate sheet as a CSV to avoid manually entering 358 tax rates every quarter.

I see that you have already integrated Avalara, and I would use them if they weren't asking for $695 per year for their bottom end package.

Perhaps you could make their hooks available for other cost conscious users to import their own rate tables.