I think the answer depends greatly on what your field is. For software developers, web developers, and others in the IT field generally, Stack Overflow Jobs (https://stackoverflow.com/jobs) is an excellent resource. You can filter by country as well as by whether or not the employer is willing to sponsor a visa and/or offer relocation support.
Since you are already on LinkedIn and getting messages about potential jobs on that platform, you might also consider putting something in your LinkedIn bio about being open to new opportunities in your desired countries, along with a note about needing visa sponsorship. "Open to new opportunities in Germany and Austria with visa sponsorship." Something like that.