You might try looking in some dedicated manga subreddits. Some popular ones are r/Manga and r/YaoiManga. Check the pinned posts or do a search within those communities.
A frequent theme is the struggle for equality. Omegas often face discrimination in omega verse settings, and many fanfics revolve around their fight for equal rights. Also, the theme of self - discovery is common. Omegas may discover their own strength and uniqueness in a world that tries to define them in a certain way. In addition, family and pack dynamics are also explored in some omega verse fanfics.
Sure. 'A/B/O University' is a great one. It has an interesting plot where an omega tries to fit into the university life full of alphas and betas. Another is 'Omega's Hidden Power' which focuses on an omega discovering his special abilities in a world dominated by alphas.
I think 'Sasaki and Miyano' is also among the best. The story is set in a school environment. Miyano is a fudanshi (a boy who likes BL) and Sasaki is a popular boy in school. Their relationship starts from Sasaki's interest in Miyano and slowly blossoms into a deeper connection. It has a lot of cute and heart - warming moments, and the way their relationship progresses feels very genuine. It also touches on the insecurities and self - acceptance that the characters go through.
Another is 'Yuri!!! on Ice'. Although it's more sports - centered, the relationship between Yuuri and Victor is really well - developed. It shows how they support each other, grow together, and their love story unfolds in a very natural and heart - warming way. The plot also has a lot of depth with Yuuri's journey to regain his confidence in skating.