Guardian (2018)

Both main characters have somebody they wish to protect and are willing to risk their lives in doing so. There are sci/fi elements mixed into reality and have very heavy undertones of gayness/touchy-feely friendship that makes you go uwu. There's also quite a fair bit of action and government manipulation going on.

Both stories are in the Crime, Detective, and Mystery genre that feature an unlikely cooperation between two polar opposites. Later forming a strong bond/friendship that even goes to lengths of sacrificing oneself for the other.

The lollipop. (you will see what i mean)

Also, both are mystery, detective dramas.

Although in Guardian the gay storyline is muted (in script, the visual language makes up for it heavily) Ossan's Love is similar in the aspect of the storyline revolving around the main character falling for another guy.

[There are no supernatural, or mystery elements in Ossan's Love]

The main story of this series is different, but similarities occur because the main characters waited more than 100 years to meet each other again.

Both are adapted from a BL novel but got changed to bromance in the making. This guarantees for Bromance at its best! The two male leads go from a cold relationship to becoming brothers for life (more extreme in Xiang Long were they are actually at each other's throats at the beginning.
If you liked one, I'd definitely recommend the other!
It's worth a watch!

Okay they left out the actual romance part in Guardian buuuut the love interest in this drama so far reminds me so much of Shen Wei that I need to recommend Guardian

Supernatural, investigation & friendship. What I love most(like guardian) is the CHEMISTRY & Bromance between the lead. Super strong. Great acting by all actor even the minor role. One house scene remind me of guardian when sw bring yl back home. This drama got an international recognition even a few drama after this reuse the evil spirit name in here.Dont worry, it's not that scary....

Same author, similar themes.
Same kind of team work in law enforcement.
Both have a gritty urban atmosphere.
Both have a friendship/romance/bromance that tries to survive in a world full of inner and outer conflicts.

Both have strong chemistry between the two male leads / Bromance

Both are detective based; The sleuth of Ming Dynasty is more investigation and crime solving heavy

Both are adapted from BL novels but without explicit BL

The sleuth of Ming Dynasty is a historical based drama while Guardian is fantasy based