> Many EU countries still allow for individuals/small companies to choose to use simpler book keeping, without going through all that bureaucracy.

Not really. Well, only if you are a really really small company (read below VAT threshold).

Once you're VAT registered the tax authorities will want the irrevocability.

There is no such thing as irrevocability in IT(unless we talk about physical retail sale registers every retail business has to have - these are certified standalone hardware devices that print receipts and store the transaction details in write once memory). At least in this EU country the requirement is that you keep the original file for the retention period and you present it on request. If you mess with it in the meantime and you get caught you're essentially falsifying financial tax records. It doesn't matter if you keep your invoices in .txt files on a hard drive or in a cloud based "irrevocable" system But if you're doing retail your accounting data better matches with the printout from that hardware device that logs all the sales.

However, having said that the complexity in doing small business accounting (at least here in Poland) is not in generating invoices, but in accounting for them correctly with regards to VAT. Is this just a domestic sale/purchase? Is it an EU goods or services purchase/sale? Is it the same but for outside EU? Is the client a company or an individual? Does he/she want to return a item. Did you send an invoice, but never got paid? (you're still on the hook for the VAT) All those things matter and affect how you're supposed to record your VAT. In many cases of cross border transactions you end up recording VAT in a special way so it cancels out, but you need to report the right amounts. The logic to not mess this up is what makes people pay for this software. There is nothing stopping you doing all of it in Excel if you want, but if you mess up... It might be expensive.

At least in NL, the final version becomes irrevocable when it is send to the customer (or supplier). Corrections are done with a new number (or more than one) Sometimes you need more than one sequence.