I'm in the same boat. I was going to send out book proposals this month...

What I've learned:

1. You need to write a book proposal like writing a business proposal for an 'influencer'.

2. You need a marketing plan since publishers don't do this anymore.

3. You need an online platform with many many followers. Blog, podcast, or videos. Twitter seems mandatory as most publishers look here first.

4. You need a PR or talent agency to schedule interviews, speeches, and other public events.

For you I would suggest hiring a PR firm first, then scheduling some interviews and podcasts. In your situation I would suggest podcasts or interviews with other small Youtubers that have philosophy channels.

Not sure what PR agencies there are in Europe but here is one in the U.S. : https://www.allamericanspeakers.com/infopages/about.php