How do you deal with contracts & DPAs as self employed (in Europe)?

Hey everyone, I'm based in Europe and I'm self employed, with my own VAT which allows me to invoice customers. For now I've been using Paddle, so customers get invoiced directly by them and I only have to deal with Paddle, which is fine with my current setup.

My biggest worry is starting to reach out to medium-big businesses that might ask for contracts, DPAs, custom invoices, and all those things, which would show that I'm self employed and not a "real business", and that might scare them away.

What's your experience by dealing with companies as self employed when selling them a SaaS?

When shall I incorporate as a real business? For now the revenue is still quite low that wouldn't even cover the business costs. And it might even be a problem as I'd be the only person of that company, and the only one working on everything, so it might be seen from the tax agency as a way to pay less taxes (and get fined).