Additionally, B2C has tight margins, so eating into those hurts a lot.
Whenever I looked at companies like Quaderno, they looked great, but when I actually tried to use them, it turned out that their solutions are far from perfect and often incomplete or simply incorrect (e.g. I would not comply with local laws in my country). This is a common theme: even B2B invoicing, which is far simpler, is not implemented correctly by companies that say they do "invoicing". For example, Stripe invoicing won't do JPK_FA or KSeF in Poland (SAF-T reporting), which pretty much makes it a no-go. Many service providers are also incapable of producing bi-lingual invoices.
I ended up using Braintree (don't make that mistake) and now I'm migrating to Stripe — but for payments only. I have my own subscriptions and invoicing, and I use a local (Polish) company that has an invoicing API to produce JPK_FA/KSeF data.
If I were to even look at B2C, I wouldn't even consider doing anything on my own. I would go with Paddle, carefully considering my margins.
I realize this is not the answer you were looking for but it's a real-life data point.