In regard to Morpheus you could use the nn Moe or Mori instead of Morph.
To give you an idea of how “mainstream” the names you have already chosen are, Osiris was ranked #1082 given to 179 boys, and Helios was ranked #4963 and given to only 19 boys in 2017. From what I could tell, Morpheus was unranked, which means it was given to 4 or less boys in 2017. Someone can correct me if I’m wrong.
Let’s see if I can give you a few you might like…
Aram (Air, Airy): Pronounced AIR-um or AH-rum. It’s a biblical name meaning “High Elevated”. If you pronounce it the first way, I think people might confuse it with Aaron. If you pronounce it the second way though the nicknames might not stick, but it sounds cute on it’s own in my opinion. Aram was ranked at #1782 in 2017, and given to only 85 boys.
Artemis (Art, Arty, Tim): Pronounced Art-tem-is. This was the name of the Greek goddess of the moon, however, it tends to be given to both genders. (Though slightly more popular for girls). The meaning of it was a little unclear to me. I found either “Moon”, “Safe”, “Hunter”, or “Butcher”… So before using this one, I’d do a little more research to make sure of its meaning if I were you. Artemis was ranked #3975 and given to 26 boys in 2017. For reference, for girls it was ranked #1520 and given to 144 girls in 2017.
Astraios/Astraeus (Troy, Astro would be cute 
) I couldn’t find one common pronunciation for these two, but I think either uh-STRAI-os or uh-STRAY-os work. I’d just pick whichever one you like best. Like Anna, some people say ANN-uh and some people say ON-uh, so it should be fine. It means “Of the Stars” or “Starry One”. In 2017, it seemed to be unranked. So again, that means it was given to 4 or less boys.
Bastian (Bash, Ian): Prounonced like Sebastian without -se. If you think Sebastian is too common you could always use an uncommon variation. Of course you still get “Bash” as a nickname out of this one. You could also get Ian. I’m sure you know, but this name means “Man From the Anient City of Sebasta”. It can also mean “Revered”. Bastian was ranked at #1186 and was given to 155 boys in 2017.
Cedric (Ced, Ceddy, Ceed, Ric, Ricky): Pronounced SED-ric. According to Nameberry, Cedric means “Bounty”, and according to Babycenter.com, Cedric means “Chief”. This name ranked at #983 in 2017, and was given to 207 boys.
Hadrian (Hade, Ian, Dean): Pronounced HAY-dree-un. According to Nameberry, this name means “Dark-Haired”, which would be kinda cute if your little boy was dark-haired. This name is also very uncommon. It ranked #3376 and was given to only 33 boys in 2017.
Lucius (Luke, Luca): Pronounced either LU-shus or LU-see-us. I added this one because it has a very similar sound to Cassius, so you might like it. It has a nice meaning: “Light” or “Shining”. It ranked #1421 in 2017 and was given to 122 boys.
Oceanus (Ocean, Owen, Sea): Pronunciation varied quite a bit with this one. The Greek way seems to be
O-kee-an-us (Though I am not an expert on that, haha). In my opinion the Greek pronunciation would sound good with most Japanese names. Other pronunciation are O-see-an-us and O-she-an-us. It means “River” or “Water”. Oceanus was unranked as far as I could tell.
Zaven (Zav, Zavi, Zave): Pronounced ZAH-ven, though if you prefer pronouncing it ZAY-ven I don’t see why that would be a problem since most people wouldn’t know- I believe a name is what you make of it anyway. The ZAH-ven pronunciation would probably work better with the Japanese middle. I did a little research and I found two possible meanings for this name. One is pretty cool: “He Who Rides Horses”. The other might be a turn off for you: “Restless”. I don’t know which is correct, but you could give it some thought. It was ranked at #5256 and was given to only 18 boys in 2017.
*Ranking source from the Social Security Administration National Data.
I can give you more if you like Good luck!