Hi @IvanPsy, welcome to the Keyboard Maestro forum!
With regards to the OCR (only necessary if invoices are scanned) and PDF thinning, neither application provides these features natively. But the other requirements you mention are in the wheelhouse of Hazel, thus it’s a great fit.
I’m sure you could do the same with Keyboard Maestro but it would take more time to develop and might require the incorporation of one or more third-party tools available via Homebrew, or otherwise.
Keyboard Maestro is an amazing and powerful tool providing endless opportunities for macOS automation. Hazel is great too, but much more specialized.
If macOS automation, in general, sounds appealing to you, and you are willing to spend some time to learn Keyboard Maestro, I suggest you download the trial (+ read: Getting Started [Keyboard Maestro Wiki]) and/or watch this video: Getting Started with Keyboard Maestro. It's an investment, but one that many, including me, find to pay big dividends.
When specific questions arise, this forum is a great resource.
BTW, as you use this forum, here's a great place to begin: Getting Started with Keyboard Maestro and the Forum - Forum Admin - Keyboard Maestro Discourse