It's really frustrating that the issue isn't fully appreciated. Therefor we decided to get this custom developed by Simtech Dev a while ago. This also greatly helps with charging 0% VAT for ICP customers. It were a decent number of hours charged and there are still parts where it's not fully integrated. I fully agree excl vat prices, but show including vat, should be default CS-Cart functionality.
We are setting up the one stop system by creating a rate area for each country. We integrated an API connection with Exact that can connect a rate-area to a VAT-code (not vat number). This will give us the data we need to correctly file our OSS-taxes.