These are all different questions than "why do you think it these are issues to overcome?". Perhaps it was not your intention, but your original question came off as challenging OP's complaint.

What measure of difficulty of content discovery are you using to determine that it is difficult?

Get 1000 random people, ask them to list 3 to 5 of their interests and to find them on "Lemmy". See how many successfully complete the task in less than 5 minutes. Do the same on Reddit.

What would not difficult content discovery be?

• A better onboarding wizard.
• An unified search engine that index all of the content, not just what is available in an given instance
• Recommendation algorithms (no, not all algorithms are bad)
• Searching people by their interests