I have read a lot of yaoi manga so it is hard to list it all but I will list ones like I would like have recommended to me.

Things I want in the recommendation
Older Seme Younger Uke
Age Gap

Junjou Romantica
Hachimitsu Darling

NO RAPE NO ABUSE BY THE SEME OF ANY KIND

________________

Hana wa Saku ka
Koujitsusei no Tobira
Abiru Junjou (student-teacher)
Soredemo Mata Asa ga Kute Koiwosuru
Karada Meate de Warui ka second story
Kyoudai-Biyori (warning:brother incest)

Others:
Ake Nure Goyou ni Furu Yuki wa (uncertain age gap)
Last Client (unfinished but I still love it).

________________
It's the Art of Time, it's the Art of Life--
Of the player and the craftsman,
Of the writer of songs, of the creator of love,
Of those who will follow, of those who led,
There's no other way in the world ahead ♥

Hybrid Child
Tonari no Kare
Ai ga Kami wo Korosu Toki
Aikotoba wa Hallelujah
Aiso Tsukashi
Castle Mango
Empty Heart
Gravity Eyes
Honki ja Neekara
Kokoro (KANDA Neko) series
Me wo Tojite Oide yo
Meguriau yo Baby
Open Air
Punch ↑ it's completely scanlated though
Sono Te no Netsu wo Kasanete
Tango no Otoko
Usagi Otoko Tora Otoko
Yume Musubi Koi Musubi
Zettai Unmei Houteishiki

as you can see I have an affection towards age gaps. i read much more, these are only the high rated ones. hope there is anything of interest for you

Just the ones that have stood out in my memory:

Kuroneko wa Shippo de Amaeru by Ruri Fujikawa - Her stuff is always hit or miss with me. This is one of my favorites simply due to the age gap and the shy/beautiful/bishonen uke in high school.

Ore ni Koi Shite Dousunda by Kou Yoneda - She's fairly popular so you might have already read it, but fair warning, it's unfinished and likely never to be complete. It's still one of my favorite BL despite only being a couple of chapters long. I've probably read it a thousand times now.

Kimi ga Koi ni Oboreru by Hinako Takanaga - Remains one of my favorite works to date. Much better than Koisuru Boukun, IMO.

Koisuru Cupid - I see you've read a lot of Akira Norikazu, this is one of my favorite works written by her. I'm not sure how we're defining age gap, but there is an age difference. Seme is older with a career/job while the uke is still only in college. The seme was so sexy, yum.

Follow - Stories 4 and 5, maybe 3. Very smutty, almost hentai-ish in some aspects. (The fifth one is my favorite)

Love Mode by Yuki Shimizu - Fair warning, art is very dated in the beginning, but it gets better much later. Probably the very definition of age gap because I thought the ukes looked almost adolescent (12-14), she seems to favor very young looking ukes sometimes. There's more than one couple in the series, the first one can be construed as rape depending on how you view it, so you can skip it if it makes you uncomfortable. Reiji/Naoya, the other "main" couple is consensual and there's a definite age gap. There's another one in volume 6 or 7. It's hard to separate the couples/stories as they're all pretty much interwoven. A great read if you're ever so inclined.

Mauri to Ryuu - First story, not sure how this falls on the age gap spectrum as the seme is a "God" but you know, looks like he's 20. Mauri looks like a young boy and I feel like I'm going to hell for suggesting it. I usually don't care for boy-like ukes because it feels wrong, but I loved this one-shot.

Papa's Assassin by Shoowa - Pretty big age gap, but it's still in the air whether or not the "papa," Daniel is the love interest. Hurry up Shoowa and finish this already!!

Ai no Kotoba o Uragaeshi by Abe Akane - Stories 5 (my personal favorite in this collection), and possibly 6 (younger brother falls in love with older brother's friend).
Love Skit by Rie Honjou
Yume Miru Seiza by Sakae Kusama - Stories 1, 2, and 4.

I'm sure there's more out there I've read, but those are the ones that have stood out in my memory.

ETA: One more, Tsumasaki ni Kourozu by Aoi Aki.