So far ive read Ancient Strengthening Technique, Heavenly Star, Womanizing Mage, Shura's Wrath, Evil Dragon against the Heaven, Otherworldly Evil Monarch, and Heavenly Jewel Change all these novels were great with romance progression, mature content, and overall story. P.S i tried Against the Gods, Martial God Asura, while these do have mature romance the MC's in these novels are really unlikeable. Thankyou in advance for any suggestions you guys come up with
if you want romance I think it's better to read/play some VN. like rewrite, or muvluv, or grisaia series. I do know some romance novel, but none with mature (read: R-18) content. I'll give some recommendation anyway: An Archdemon's Dilemma: How to Love Your Elf Bride. I think the romance progression and overall story is good. UchiMusume. the romance wouldn't start until later volumes (when the female lead grow up a bit more), but the story should be good. Gamers. well, it's a romcom I recently read, not currently translated though, and it's nowhere near mature. but it's funny, so I'll put it here just because I read it recently.
Try the works on Andur on Royalroad. Not fully shown sex scenes but damn the romance is really good because : it's kinda a quick transmigration series. Most of the books show one life of MC in different world with different personality. Bonus point, it's also the same thing with FL. There is also a third wheel but i won't say more. Try first "Beyond?" it's best masterpiece, really fluffy, just try to not be too sad because of 2 minor deaths. Call me when you need the chronological order of the books. Edit: Did i forget to say in Beyond?, MC is a mad scientist (heck he is even repulsed by himself for creating a sentient robot), there are quite epic moments, good fights and a really likeable FL with a dragon tail ? It's a condensed of awesome fun. Also it's double isekai, isekai MC goes into another isekai (kinda like MC from Cheat from lvl 2). And the parents are for some reason not repulsed by knowing that their son got the soul or memories from another person.