Very nice write-up. We had the same requirements and went through the same stages and similar conclusions. Though we opted for #2, using Stripe+Quaderno, to create us tax reports. The VAT submission and processing by our accountants has been trouble-less. A bit extra paper work (well, PDFs) but the accountants do most of that. Quaderno writes and emails the invoices and there's some issues with customization, never mind some users get confused seeing both Stripe and Quaderno URLs. Automated testing is a challange with two API involved, we found mocking responses to be impossible, and both APIs keep changing over the past years. Starting fresh in 2020 we might have gone with Merchant of Record, too.

> Note that VIES is a government system that has been known to be down for maintenance

Way too often. And each country has different maintainance windows.

> Post-Brexit, I guess that the UK will be treated as if it were a non-EU country

That's our assumption, too.