A small amount of hopefully useful feedback from someone who purchases business software:
1/ Your website gives a 'open source hobby app' vibe over 'professional, dependable software business'. Selling support contracts in particular requires some expectation that there's a solid business standing behind it that won't just be gone tomorrow.
2/ The 'redirect to an external store to download / purchase' will probably be making any sales funnel leak like a sieve. There's a complete discontinuity between the design of the two sites, and along with trusting you with their information / card details, they now have to trust a random third party.
3/ For a ~€1000 piece of software, I'd expect at least some period of eg. email support, rather than just supporting through Github issues.
4/ I'd remove the option of having the open source edition available through sign-up and just link clearly to the latest release. If you're struggling to even get any interest on the commercial side, getting it out in front of as many people as possible would be my priority. A clear and unambiguous 'Get started' section that doesn't worry about open vs. Pro is probably a good idea too. Worry about converting those to paid users later.